Time to show you where you're wrong:
1. "Ok, the knife example is valid. Guy per guy, chaos marines are cheaper than plague marines. You CANNOT argue against that unless you have no concept of numbers. You are talking about making marines expensive, which yes they can be if you give them a mark, CCWs and bolters, special weapons, a banner, and a champion with weapons. Here is a solution. DONT. Take the marines, give them the special weapons, and maybe a mark and free CCWs if you wish. Done. Cheap."
No, not cheap. A minimum squad of Plague Marines using Plasma Guns is 150 points, and the minimum squad of Chaos Space Marines using Plasma Guns is 170. Where did this "cheaper" thing come from on your end? If you plan to just use a 5 man squad of Chaos Space Marines with a Plasma Gun, well, good luck with that actually doing anything when you could have just gotten Cultists for cheaper.
2. "You always seem to feel like people are saying plague marines are bad. Not a single person is. I have no idea where you get that attitude from. We are simply saying they are more expensive and have more special rules than is required. Key word, REQUIRED."
I already showed how they are less expensive, and having more special rules as a complaint is simply stupid, sorry. Anything regular Chaos Space Marines can do, Plague Marines are doing better, and then if I need something cheap to hold an objective I got Cultists.
3. "Plague marines are a Bowie knife because they aren't always needed to complete a job. You can cut a piece of cardboard with a properly sharpened Swiss Army knife just as easily as you can a Bowie. So why take the bowie when it's more than what you need?"
Plague Marines, as I showed by the better efficiency and points value, are a swiss army bowie knife, and Chaos Space Marines are trash.
The reason you take Plague Marines is because you pay for what you get. Compare buying a more expensive pair of boots to a cheaper pair, and you'll notice the cheaper pair sucks because you need to replace them, get them repaired, etc. Looking at this further, Plague Marines are an expensive pair of boots you got for cheaper than the bad boots.
4. "Also, plague marines are STUCK with mark of nurgle. They can't take a different one. Sometimes, a cheap ccw unit is what you need, you can get more attacks from csm with Khorne and free CCWs than you can with an equally priced plague unit. And guess what? Those plague knives aren't always needed to win combat. Fire warriors and Termigants really shouldn't be such an issue that poisoned is needed."
You're complaining they're stuck with the best overall mark? Wanna complain about something else that's stupid?
A cheap close combat unit? MoK, and all free CCW's at minimum is 85 and they won't reach combat. 20 more points gets me Khorne Raptors, and they have mobility that the former lacks (all while being able to take 2 Meltas at minimum, which is awesome). As well, Poison guarantees rerolls to wound on those two targets you named, which is definitely not a bad thing.
5. "Also, you are saying that plague marines are troops with a nurgle lord. Did it ever occur to you that not every csm player takes a nurgle lord?"
I always said that scoring is merely a bonus, and being able to simply take Plague Marines, already the better unit as I shown, means I get to spend more overall on shiny toys. As well, not taking a Nurgle Lord is silly when it's the best mark. Not everyone takes it, true, but it's the best choice.
6. "Because in my csm army, plague marines aren't scoring. But csm are. My scoring cult unit is noise marines. And csm are miles ahead when it comes to popping 14 armor, and are cheap enough that I don't care if I sacrifice them."
Once again, scoring is merely a bonus. Since you used you Troop slots on Noise Marines and not your Elite slots, you can bring Plague Marines to do the job that the CSM's are doing, except that Plague Marines will do it better.
Emphasis mine. Mathhammer says No. 4 cultists are the same as 1 chaos marine against bolter fire. Against anything strength 6 or higher, then 3 cultists are equal to 1 chaos marine, assuming AP4 or higher.
I'll give you the argument where you're right (ie: plaguemarines), but cultists are grossly inferior to chaos marines. Their only redeeming quality is that you can take them as 50 point, 10 man squads, minimizing your "troop tax". And hopefully having them never get shot at.
According to some, there really isn't much point in making more than one of anything, as there can only ever be one "best". One "Best in Slot", one "Best at Function" (ie: melee, shooty, etc). Now, they may not all be equal (wraiths to praetorians), but sometimes it's nice to have options, even if one is marginally better than the other. But if you're going on the "Best is the only option" train of thought, then expanding a codex will always lead to more "duds".
Which, getting back to the topic (though I'm not against tangents), nobody is arguing that Plagues Marines are better than Chaos Marines, but the question is: how much better? Are chaos marines 95% as effective as plague marines? Is it enough to matter that much to most people? And that's usually where you'll see me in these "arguments". Sure, there might be better options, but it's not enough to matter in the game most people play. So, if you want to take "sub-optimal" choices, you can easily overcome the difference on player skill, if not other aspects of your list. Moreso if your opponent is also on the same page.
To Possessed: I will say that "Fleet" means a lot to a guy like me, and that's something they provide that nobody else does.
Guess I did the Mathhammer wrong. Whoops, not sure what I did...
When it comes to options, I can look at the Necron codex and think to myself that Praetorians, Shards, etc simply don't exist and you realize how few duds there are. In the Elite slot, Flayed Ones (doing the Genestealer infiltrate thing but don't die to a breeze), Lychguard (Assault Terminator equivalent give or take), Deathmarks (being the best Elite), Triarch Stalkers (army enhancer and can kill things in one shot) and the Tomb Stalker (monstrous creature edition but doesn't suck like the C'Tan) all have a specific niche they fulfill. Wraiths end up taking over the one that Praetorians fulfilled (elite jump assault unit), and then I could go over the individual Fast Attack units if you wish. When I have more options, it simply means more models for me to use. Otherwise, I can easily find something else to do the job.
So when I look at the Chaos Space Marine codex, we just end up with so many duds. Berzerkers end up doing the same job as Possessed but better, and simply put Fleet isn't worth the price, lack of grenades, and MoK, when I need to buy a Land Raider for both anyway if I expect them to actually reach combat...
When Plague Marines are doing the same things for less, points and money wise, I'd be insane and unreasonable to use regular Chaos Space Marines. It's as simple as that. Player Skill won't matter on that when you're building an army inefficiently, otherwise an all-Genestealer army has a decent chance of winning, and both you and I know that's not true.