I've only experienced problems with converting a stereo MP3 into a mono one with maybe one or two songs, where it ended up with static or one channel drowned out the other. Personally, I think the mono podcasts sound fine. Better than AM radio, even. Stereo MP3s often sound worse, in fact, because you split the channels and each channel has a lower bit rate. For example, an 80 kbps stereo MP3 is actually 40 kbps per channel, which is why you hear the compression artifacts in the music (that swishing sound). We would have to rip our shows at 160 kbps stereo to replicate the sound of our 80 kbps shows, and what would it gain you? Well, for one, it would take you twice as long to download each episode and take up that twice as much room on your hard drive or iPod... and it wouldn't necessarily sound any better! Also, the podcasts aren't intended to include pristeen copies of the music... FREDDI is right. We'd rather folks go out and buy the records, CDs, or downloads from the bands if they want the best quality sound. So think of the podcasts as more of a sampling of what's out there rather than an aurally perfect copy of it.
By the way, ALL of our shows are supposed to be ripped in mono, so that the audio is consistent when listening from one program to the next.