Why are there respawns in 5 mans?

Having the 'trash' mobs respawn can help keep a group focused on progressing and beating a dungeon. In general they don't come back quickly and give you the time you need to get through the place with several mistakes, but some situations may be more challenging.

Q u o t e:
Well I was and I thought it was ridiculous that i was punished for trying to complete all the bosses of an instance.

As far as Blackrock Depths goes, it is a very big dungeon and while I haven't been in there in a long time, I believe you can still complete it without dealing with respawned mobs as long as you do things in the right order.

We do pay attention to the content that we implement though and we are always aiming to build upon this and improve the new content that we make.