As a prospective Live user, if mp3 support deters any Ableton resource or technician from advancing Live in relevant matters like 64-bit support & optimization, automation curves, etc. then I say:
NO mp3 support.
As an actual dancefloor assistant and as a sensitive listener I say:
No mp3 support.
Mp3 works well on small sound systems where speakers cannot handle the entire freq range, or for cataloguing or previewing purposes, not for large amplification or for middle to high fidelity systems, like in parties or dedicated listening rigs.
Live is mostly for Live amplified performances where mp3's deficiences are going to be easy noted. If you need low-fi in your playback then add a specialized plug-in.
This whole thread shows that many outhere still need to come to terms with the new levels of resolution that amplification sound systems (and Dj mixers), actual soundcards better signal to noise ratios and plenty oother gear around... are actually bringing out already.
Soon old mp3's emperor lack of clothes is going to be globally revealed (as it happens nowadays in most good Discos), particularly when being bis-a-bis compared with better sound sources (like in sequencial Dj sets). Be warned.
And about exporting pre-mixes for judment... on mp3??? what is it going to be judged? commercial appeal? length? estructure? Nothing close to mastering qualities, I assume.
Today storage capacities and internet bandwith increase exponentially. Professional need of mp3 if exists is actually diminishing on the same rate.