Releases can be nuked for all sorts of reasons just like they can for all other areas of the scene. Two main general reasons:
I do not have a list of 360 scene rules to hand but generally
if a group repacks a P2P release it will be nuked (even if the release is good from a technical standpoint)
sometimes prerelease games will be nuked if they do not come from a retail source (although it can be weeks before we can tell this).
Some nukes I have seen are for lack if compression (you waste some bandwidth).
If a group propers another release unnecessarily then it will be nuked. Outside of the topsites mind you will probably not see these if it is caught early.
Some nukes are for naming errors (again assuming you get the correct version then there is no problem) or dupe names.
Some nukes are for lack of NFO (not nice but not fatal by any means).
While you may think this would be fatal you can sometimes fix releases (you can stealth releases).
Others can not be though (X-Blades_PAL_XBOX360-iCON was recently nuked for bad video data) and you will need to grab the redump (or a patch if it exists).
Naturally some can have elements of both.
If you are on abgx then if you hover over the nuke symbol it will give the reason.
As for why bother, the scene can best be described as a race and sometimes checking is not as thorough as it could be. Sometimes groups do not have a clue, sometimes it does not matter (many do not care about being banned from live and if that means I can play a game 3 weeks ahead of time then by all means release it for me to play).
Regarding lack of a proper if it is just a nfo error/similar then a nfofix may be released or a dir fix may be released and it will not be reflected in the list or they simply may not bother.