Near and Mello were stupid and unnecessary characters- they had way too much deus-ex-machine on their side. Near didn't deduce who Kira was- his eyes just lit up and he magically guessed many things and ended up being right. It should have ended with L.
after watching the anime (didn't read the manga) i will agree that they were unnecessary charas.
but with L dead, there had to be someone to pick up where he left off. IMO it was fitting that L died, it was just going to end up that way.
another problem with Near and Mello, is that there personalities were just rehashes of L. apparently you have to have some kind of OCD to be a detective.