Maybe it's just because he's in Sacramento (I don't know a ton about Evans), but his point, rebound and assist averages have dropped every season, and he's a career 25.6% 3PT shooter—personally, I don't want two crappy 3PT shooters in Boston's backcourt. Also, he's averaging only 4.5 assists this season and 5.3 for his career—not good numbers for a starting PG, in my book.
So he's not a good enough shooter to play the 2, and he's not a good enough (or willing enough?) passer to play the 1. So what do you do with him? He seems a bit short to play the 3.