well actually that question is really simple.. Sure when you look at spearmen you are thinking i can get the same amount of attacks but no.. u cant. Here are a few reasons why you cant
- When down to 1 rank corsairs still have same amount of attacks
- On the charge Corsairs have same amount of attacks.. you cant double rank attack with spears on charge.
- If you are fending off ppl from the front and the side at the same time, you wont lose any attacks.
Also.. you get +1 save from missle fire. Corsairs can take magic banners. Model looks cooler. haha...
Now personally I still use big block of warriors over big blocks of corsairs cause you save 40 points per block. In my eyes i still like corsairs more for flanking with 6x2 or 5x2.. They are few numbers so panic easyer but the +1 save from missle is nice and its likely they will hit 1 rank so still lots of attacks in the front.
I like to think like this.. if I take 2-3 big blocks in a game I will save 80-120 points and that will be a small unit of corsairs to flank. In the end it matters on how you play and if you think the 40 points are better or the +1 missle save more attacks in situations and being able to take a magic banner.
As for the take all small blocks of 10 or so, I think its a very bad idea. We move 5 but its not enough to ruin an armies movement totally. Also you will panic like no tomorrow.