[MicroBob]

also - when he does call there is still the chance that your ace outs are good.
and certainly the chances of a rivered 5 leading to a split-pot are decent as well.

so on the river there are 7 cards that you're reasonably happy to see (I wouldn't go so far to call them 'outs' though because you really have no idea where you stand)

this isn't THAT much of a factor...just something that occured to me....that you aren't COMPLETELY dead if he only calls.
But the original idea is that a bluff here can still win the hand....and checking almost automatically loses no matter what he has.