The purest argument for me is: think about how important email itself is to the company. Answer should be very, and if not... then there is the real problem!
Other comments here have covered it re: bugs, deliverability, trends, legislation, but I would add:
Being a leader not a follower - specialists are more likely to innovate, a follower will not try new things. This is very important in the long term.
Good luck :)