I play the lossless at home on the big stereo over a high speed network. The 320kbps stuff I either stick on a memory card for the phone/car (where I'd rather have masses of musical choice than high quality stuff) or stream over the internet, so I don't want to use too much of my data allowance, or have stuttering/buffering when I have a poor connection. Same reason my film collection is only being done in mp4.
I could get the server to do the conversion to a lower bit rate whilst I'm streaming, but the processor is a bit rubbish, so it's easier to just have the right version there.
And my ripping is being done in parallel, rip lossless version, then use that version to make the mp3. (While I'm ripping the next CD.) So I've only got half my collection on lossless too.....