As more local stations buy audio switching boxes such as Optimax and Neural encoders, the 5.1 signals we see on our home receivers don't mean the channels are actually being used. Stations have problems switching between genuine 5.1 sources (such as HD network shows) and all the other content that originates in stereo. Octimax and others send out everything as 5.1, eliminating the jarring audio switches between sources. These boxes pass through real DD 5.1 tracks and they re-encode stereo sources as synthesized 5.1. The net effect is that stereo material often ends up almost completely in the center with only music in the other channels. Some well mixed stereo material will sound pretty good - this year's Rose Bowl, for example. Other material often ends up sounding mono. But, at least the voices are in the center channel, which is probably better than stereo for talking heads.
Local station issues aside, there are also huge variations in the quality of the 5.1 mixes. Some shows and sports production crews care about the 5.1 mix. Many others don't. They're happy if it sounds good in stereo, which is all the vast majority of people hear, even many of those with HD sets. Kind of sad.