The most interesting thing with boss fight is not the boss fight in itself - it's the fact it's a cheap but nonetheless working way of telling the player "ok, you have completed this section as far as the main story is involved, so now go back and try the road that just opened".
Sure, the mechanic is a bit obvious, but it's a real way to just let the players know they have reached a milestone. A big boss fight in a closed room.
Now, I tend to agree I don't remember the boss fights to be that epic nor that interesting. Actually, I remember Cyseal last fight as I was playing a stealth ranger and had to cheap-shot him while my pal was getting wrecked in melee. I had to lure the adds while at the same time being sure they couldn't attack me or I was dead, and... It wasn't very fun.
But, we knew we were done with Cyseal. A few loose ends to tie, but now we were actually, really done. Milestone.
