I don't think that it is a matter of "limiting" God. I guess I've never heard anyone say that the theory is invalid because it "limits God".
The primary reason for rejecting the view is, for many, simply a matter of the traditional method of understanding Scripture, and it's origins.
Put simply (perhaps too simply), it is more likely that the people who actually lived in ancient times and lived through the events of those times had a better understanding of those times and the events of those times than someone who lived hundreds of years later.
Based on that reasoning, it's simply more plausible to accept the traditional view of the authorship of the Pentateuch than it is to believe a theory derived hundreds of years after the fact.
Whether the tradition started at the time of Moses, with those who were actually in the wilderness with him, or if the authorship was attributed to him later, the argument still remains that Mosaic authorship had been accepted by those who were much closer in time to the actual events than the later critics.
Another reason for rejecting the theory include the fact that it is only one of many theories. (I know it's not the best source, but Wikipedia has some good information on these)
This may be a bad basis for faith, but on some level it is simply easier to reject all of the competing recent theories and hold fast to tradition. (The Gordian Knot approach.)
When it comes down to it, for me anyway, whether Moses wrote the books or not isn't of the utmost importance. What is important is whether or not I trust that the Scriptures are actually "God-breathed". Since Jesus Himself quoted from the books of Moses, He clearly accepted them as Scriptural. That's good enough for me, so I don't worry about such issues.
Taking the next step, I personally reject the theory simply for the reasons above, but I wouldn't get into an argument over it. I reject the idea, but accept that I could be wrong, and that God could have easily given us exactly the Bible He meant to give us in any way He chose. If I'm wrong about the authorship it doesn't really matter so I don't worry about it, so it's really a non-issue to me but if pressed I'd say I believe the Mosaic authorship.
I suspect that many who reject the theory do so for the same reasons.