I use lossless for two reasons. I can be confident if I burn a CD containing lossless tracks they will be exactly the same fidelity as the source, on any equipment for any person I might give it to. The second is lossless can't render your music obsolete. If a more efficient codec is released, simply uncompress your music and recompress it with the new codec.
As for transparency, it's more than likely Musepack or LAME insane would be transparent on 99% of my material. That said, I have a hell of a lot of hard drive space so why settle for transparent when you can have mathmatical perfection? Some would then probably ask "Why compress at all?", to which I would reply "Why not?". It takes all of 5-10 minutes to rip a CD to Monkey's Audio with extra high compression, and a 3.12ghz processor can handle the playback CPU usage levels fine.
If you can save space by losslessly compressing without any significant drawbacks then there's no point leaving music uncompressed.