Might just be a terminology thing, but I don't think mastering engineers work off of tracks. I think they're given mixdowns and adjust from there.
There are some fascinating isolated tracks out there, but I wonder if knowing that stems will get released will push things in the studio even deeper into the territory that every recorded note must be perfect. Making a record already produces a lot of self conscious anxiety that affects your decision making, so knowing that people will also be listening to and analyzing your individual tracks is even worse!
Stems is only four tracks. That certainly works well for "standardized contemporary music for DJ" but it doesn't sound it is a universal format for all kind of music.
