Because it's seeking to tie those involved or who it chooses to be supportive of to the status quo, of the state - the very same state that supporters accuse of being racist. The larger point is that this, if succesfull, would contribute to stifling any more radical movements growth, by demonstrating that the state 'works', it isn't racist, it's 'fair' if only you get involved - that there's only way way to go about things.
The developmental aspect you mention, that this could be viewed as a wider stepping stone to deeper questions being posed at a later date - well, sorry i just don't think so. The same arguments were made when w/c people were first co-opted into Parliament. The idea is right, but this one is happening on the wrong terms. There's no need to begin by endorsing the status quo - in fact, i suspect that would be more likely to put more people off - except a few aspirant profesionals or careerists.
It also serves to funnel political anger into race-based moulds rather than wider community based actions, which are almost inevitbaly going to be cross-race in many areas.
There's also the fact that building up a buiffer zone between real power and the people below, a buffer zone made up of the beter off elements of those 'below' is an absolutley classic tactic of power.
I've got nothing whatsoever against initiatives to include, encourage or help black people into more political or community involvement - the key though question is - on what basis and on who's agenda? Who's doing 'the educating?'