Downmixing is a capability of the system/decoder. How well something can be downmixed is also a matter of how well it was mixed in the first place. If you play a discrete surround mix (like DTS/Dolby Digital/SACD/DVD-A) back on a stereo system, it's up to the player to figure out how to downmix everything. If the engineer doesn't take in to account how a surround mix will fold down into stereo, it's the same thing as only mixing on studio speakers, and never listening to it in your car.
Also, my guess would be that most music surround mixes are just 5.0, and they're not mixing a discrete LFE channel, but rather letting Bass Management feed the sub.
I think it's clear there's not a lot of surround mixing going on in music. Just take a look at the SACD/DVD-A row in your local music store. I know there are also DTS-CD decoders out there. Surcode makes a lot of surround software encoders. Of course you need a decoder on the other end.
I think it would be great if SACD or DVD-A took off, but it's pretty clear that people would rather have music on their cell phones, than high sample rates/bit depth and discrete surround. sigh.
The other place surround sound is showing up is now for HDTV. You can actually broadcast Dolby Digital over HDTV, so both shows and commercials are getting mixed for surround.
What I also find interesting is that on many DVDs even if they only have a stereo mix, it's been encoded as a Dolby Digital 2.0 channel. This seems to be the common practice even though you can put a stereo PCM stream on a DVD. Well that's video guys for ya, always hogging the bandwidth.