Hi Ted. You are correct about MP3 not being as good quality as wav however it is the track you get back that is important. Most musicians can work with an MP3 work track BUT lay down their important parts as a wav.
Usually because of the huge size of a wav compared to MP3 it cannot be sent as an attachment to an email. Snail mail is a slow laborious method of exchanging worktapes so many people use MP3.