This is a WAG, so take it with a grain of salt.
I would think that the higher sugar content of wine must helps to get rid of the bugs. Where beer it typically lower sugar wort, the bad bugs from the grapes might have an easier time taking hold.
I would echo the above posts as well. The reason we get yeast in packages it because it make consistently good beer. You could get some great wild, or some terrible stuff. It is a risk you will need to take.
That said, you could try to make a yeast starter with the grapes, decant and pitch. That way you would know if you were getting the bad bugs with it, and not have the grapes in your beer.