For a number of reasons:
Its hard to see what is going on with teams etc. no aoe indicators such as pantheon or brand's aoe's are shown in colour, but rather just gray circles. I also usually mentor my team if i die as the support by telling/reminding them who to focus, and it is harder to notice who is low hp etc if they don't have colours.
Basically i started wanting this because i played World of Tanks, and... the post mortem mode in that is really really bad. made me realise that the league one isn't much better
edit: it also makes replays and stuff like that look really bad if the person who views it is dead at the time.
[size=1] someone else freaking bump [/size]
AS JARVAN SAYS,
TODAY IS A GOOD DAY TO DIE
( in black and white :| )
we all want colour!
But then how do new players know immediately that they're dead?
riot pls ;n;