One big thing in fav our of getting Dier is his nationality, like it or lump it, I feel it is important for us to have a strong English (British) contingent of players, unlike Arsenal and Chelsea it has been part of our identity, not something which should be disgarded lightly.
Looking at next season we have Rashford, who may or may not be a starter, but will probably see reduced playing time to last season, Carrick who is winding down, Smalling, Jones and Shaw none of whom are guaranteed to be first choice, most likely not if Bailey an Lindelof are fit, and if rumours are anywhere near accurate that we are on the hunt for a new left back.
Dier can be a very good player, not sure he has shown a consistency to hit the heights but he is still young, it is reasonable to think he will improve, he is also pretty much guaranteed an England spot.
Marketing wise and for the moral high ground the more players we get into the England team the better, right now there are not that many great English players out there that would improve our squad. Has Dier reached the pinacle of DM players yet, no, but he has potential, does he improve our team, yes.... and £50m would probably be mostly offset by the decent image rights we would gain.