I have a Pioneer Elite Pro-730HDi. In the manual it states:

"The RGB input jack is designed for use in connecting a digital
tuner with RGB signal output, and it should accordingly never
be used for connecting to a personal computer or other device."

Any idea why a computer shouldn't be connected to the RGB input?

I'm not a gamer... just planning on building/buying a multimedia PC with dual HDTV tuners, DVR functionality, etc.

There is absolutly no reason a computer can not be connected since the TV has absolutly no idea where the signal is coming from.
Just make sure that you send a resolution over the RGB interface that the user's manual says it will accept over that interface,
Most HDTV user manuals contain similiar verbage about their different interfaces and they do it because they do not accept all of the resolutions that a typical PC monitor might use and they don't want you to complain or possibly cause a probelm with their receiver chip when a resolution you send is not accepted.