I've been using a unit of lesser daemons for a while now in a competitive build (I like going off-base, what can I say?). For me, I have 3 units with icons to summon on, 2 of which are chaos Glory marines where the icon is already worth the points for the morale, and one off some plagues. So even including personal icon prices, the unit is dirt cheap.
They see 2 primary uses:
1 - sit back on an objective away from the front lines, prefferably in cover where Going to Ground gives them a 3+ save. It's as good as or better then power armor vs most shooting. Just try to keep them out of sight as much as possible and let them rack up the extra objective for you cheaper then basically any other effective scoring unit available to us.
2 - Support a front-line unit in an assault. They'll die horribly, but those extra attacks and bodies can make all the difference in breaking the enemy lines. Especially against enemies like guard that fold like a wet paper bag in the face of 15-20 MEQs charging them.
Now sometimes there's a squishy target around they can take on thier own and more power to them when the opportunity presents itself. They've also helped ding up heavy artillary with it's weak open-topped av 10 armor vs assaults. And best of all, they come in where you need a hand, making them very flexible supplemental troops. That flexibility is a big thing for messing with an opponent's plans or stacking the deck for your own. I can't stress that enough. Suddenly having 8-10 extra troops just show up where you want them without scatter can really mess up the other guy's day.
When the present codex came out, I was thoroughly disgusted with them and couldn't see a use for them. Now, I love the little buggers. Having a squad of 8-10 on call is a great boon to my army.
All that said, Piling all your summoning eggs into 1 biker basket is a massive mistake. One of the defining pros to LDs is the flexiblity in summoning locations. If you plan to field 3 bike squads with icons, or pile icons on troops / HQs, then ok. Now, a greater daemon from a little 3-man squad, that sounds like it might work at first. You could keep them hidden first turn or two and then turnbo-boost out. The catch is that they'd be silly-fragile and you want them near the enemy for the greater daemon, but if they get in close, they're about to be toast. Furthermore you have to move them into position a turn before the greater daemon shows up, but you don't know when that'll be so you need something that can weather some punishment if it has to. Bikes can do that, but the price tag goes through the roof to do it.
I'd favor the chosen with a rhino or even a terminator champ if you're fielding them. Something with durability that can get in close to drop the greater daemon where it'll do the most damage. I just don't see bikes pulling this off with a price-tag in the useful range.
Might be possible to try a super-ish unit though. Biker squad with naked champ and a biker lord / sorcerer tooled up for assault. It might work...