It's a speed/weight versus vision thing. The slower movement lets you still get a healthy amount of vision checks. Personally I use -51 on this guy:
http://goallineblitz.com/game/player.pl?player_id=2354017And tbh, and this is purely imo, at high elusiveness players often become TOO elusive with a lot of wasteful movement. It's that weird dynamic where some of the fast 160+ speed backs would be better off on the power side of the spectrum.
ETA: Likewise GLB path-finding for ballcarriers is pretty mediocre.