Hmm good question. I do know that some black men did serve in some militia units obviously with whites so they did not seem to have much issue or theyd have not allowed them.
Here is a link to Wikipage
http://en.wikipedia.org/wiki/African...olutionary_War
about about African Americans serving during the war on both sides.According to it only salves where barred from serving not freed men.I am pretty sure I read this in a book about early American wars but was not 100% sure on it from memory alone so your statement about no blacks being allowed is not 100% correct and it seems it was up to the state militia.If they had allowed slaves(some Northern states promised freedom for service) it probably would have a been benefit to some extent I'd think.Either they would have been freed or they would have used their military training to resist(more likely the later) or the general population would have become less supportive of slavery. OK there was a full ban placed eventually but it did not apply to men already in.
Also Crispus Attucks was mixed African and Wampanoag so he probably was never a slave.They just like to say that in official history sources another thing I tend to forget.(I am a bit weak on this time period to be honest not committed to memory as say WWII is)
Should they have been given a worthy role or just fodder? I think they where used in most every role from infantry to spying and everything in between.