FWIW, I flirted a bit with 24/96, but decided that it wasn't worth the extra disk space needed. 24/48 suits me just fine, and if I need 44.1, the SoX resampler does a wonderful job. Unless my target audience included dogs, cats, etc., (or I was doing scientific or engineering work that needed ultrasonic frequencies) and my entire recording chain from mics to ADC accurately handled ultrasonics, I don't feel that I could really justify the extra overhead. Then, even if the recording chain was ultrasonically accurate, what about the playback system? Let's just say I'm more in the
Audio Critic camp than the
Stereophile camp, and worrying about ultrasonic accuracy for music would just be a way to relieve one's wallet of extra cash.
Regarding MP3, I'm not sure the concept of "bit depth" is relevant. LAME will happily digest a 24-bit source file and give me an MP3 that plays fine on any MP3-compatible hardware (assuming in VBR mode that the player supports VBR, as anything recent should). In any case, 48 kHz is a valid sample rate for MP3, and I haven't heard of a player not supporting it.