First off is the issue that it encourages people to spread out way too far to the point where they may as well be playing alone.
The other issue though, and perhaps the more important one, is that the game simply isn\'t made for high speeds currently. And objects moving faster then a certain amount will break the game\'s sector system. Anything higher then 100 starts making combat a big pain, and anything above 150 or 200 makes typical servers start getting screwed up.