Personally don't think it would be a good move.
The best thing for Phinney is to spend a year, maybe two, learning, developing and improving and building up experience in shorter stage races. A Grand Tou in first year as a pro is a lot to ask. As tempting as it must be for BMC to throw him in at the deep end, it could have an adverse affect on him.
I don't see how he will be of any great assistance to the Evans in terms of the GC, with the obvious exception of the team time trial, but even then it is not a long TTT and BMC have more than enough riders capable of producing a good performance on that stage.
There isn't even a prolouge that they can target this year. It would be a bad move, it would be very doubtful that he would last the distance and like I said would provide Evans with minimal help for GC.