Too much delay, too much CPU power for a mix session (for LE). It sounds accurate, but when you look at all the tradeoffs between something like a focusrite EQ or waves renaisannce EQ, it's just much more affordable to use those EQs in a mix session.
I would say yes, because that's what it was designed for, but both EQs have different sound properties, so I wouldn't completely throw REQ out for mastering.
However, 90% of the time, I would say LinEQ is the prefered choice (of those two).