This is both a simple and difficult question to answer. On one hand all fans are equal. But on the other hand I would say match going fans and those with a local are connection are arguably what made the club what it is. They're not more important, but they are needed more to keep that connection to the local area, hold the local traditions and represent the club in person. Without "local" fans (I use local loosely since everyone is scattered to the winds) there would be no real derbies, and them rivalries with Liverpool, Manchester City, Leeds etc wouldn't exist, which removes some of the allure from the club. This is even an issue an issue with players, as people like Keano have previously stated.
Manchester United, in my opinion, have always been a club with strong roots in the British Isles, as in Britain and Ireland. While true for most clubs, it always seemed bigger with United. The best managers both fostered this connection and drew heavily on it to find and maintain their success. This success lead to the popularity further afield, which introduces those fans worldwide. Some of whom now are far more invested in the club than people who can see the ground from their window.
Match going fans for any club are the lifeblood, without them and without a connection to the local area the club will be soulless and may as well up sticks to another city where it is easier to attract players and find a bigger market. Without the overseas fans revenues would slip and the club wouldn't be in that top echelon that allows them to compete finally.
I do think the overseas fans are not one homogenous lump though, there are fans who buy a shirt just because of the success, and then there are fans who get to one game a year, watch all games etc. Maybe there are 3 classifications of fans. The "locals"/match goers, the overseas/remote fans who try to get to games, follow closely, or those who just buy a shirt and watch the occasional match to share the success vicariously.
I suppose one thing to consider is that if the bubble ever burst, which fans would stick around and keep the club afloat? I would suspect it would be a mix of the local, and some/all of the second category. Who'd be more important then?