There are three reasons or more reasons to have vision at a decent level for DEs.
One reason is to defend the screen pass:
http://goallineblitz.com/game/replay.pl?game_id=878767&pbp_id=10293108MPHD used absolutely no screen defense besides having high vision LDEs set to -100 aggressiveness for season 12...and only one team managed to hurt us with screens. This allows you to focus your defensive alignments around other threats.
Another reason is to get a good jump off of the snap. Decent vision combined with Jump the Snap and First Step leads to a DE with explosive behavior off the line (assuming you have high speed and agility, obviously).
A third reason is the combined positive effects of vision on your ability to makes tackles, shed blocks, and make tackles while being blocked. These are incremental effects, but they all add to your performance.
There are some very good DEs with vision in the 50s (even in the WL), but most have vision of 60 and a few have gone higher than that.