It matters to United because of our tradition. We've always had great players in the team from Scotland, Northern Ireland, Ireland etc and I'm proud of that even though my preference is obviously for local lads from the North West. Think of 2 of our best ever players - Law and Best. Think of our most decorated player - Giggs. Think of our (probably) greatest captain - Keane. Not to mention many other great servants who've provided loyal service and memorable moments over the years such as Crerand, Albiston, McClair, Fletcher, O'Shea...
What I think is really fantastic is that, for example, a young Irish lad coming through will most probably dream of playing for Manchester United (or Celtic, Liverpool of course) due to our historic connection.
In the end it's good to have a balance in the squad. My favourite players over the years I've been watching have probably been Evra, Ole, Scholes, Keane, Giggs and Cantona. Foreign and British alike. I guess all things being equal I'd probably want something like one third Manc, one third British/Irish-but-not Manc, one third foreign. But obviously if a Zlatan, Cantona etc comes along, you shift an Alan Smith out the way to make room, so it will never be exactly how you'd imagine it in fantasy land.