Without the difuser you would have alot of micro bubbles in your tank.
If you are running a sump with a bubble trap , then the difuser is not needed.
It would be ok to use without the difuser in the tank if you dont mind the bubbles floating around.
Check this out
http://www.canreef.com/phpBB2/viewtopic.php?t=16355
or this
http://reefcentral.com/forums/showth...5&pagenumber=1