[BugsBunny]

I'm not Ed but I'll take a shot.

At best a 2 gap backdoor straight straight is worth maybe .5 outs. It'll only come in 1.5% of the time.

In this case at least some of the time when the backdoor straight comes in you'll also have a flush. This lowers the value of the backdoor straight even more. And, in other cases (as you've alluded) you'll get your backdoor straight and lose to someone holding a 6. In additional cases you'll get your straight but lose to a flush (if one of the cards is the 3 or 4 of spades).

Adding all that together I think that the backdoor straight adds so little to your hand that counting it towards your outs is basically meaningless. Without doing any math I'd guess that it's worth maybe .2 outs in total (at best .3) - which isn't really enough to sway a decision one way or the other. Since it's all estimates anyway adding .2 to an estimate of 4.5 outs still leaves you with 4.5 outs (going down to the nearest .5 outs, which is all that's practical in a situation like this, I think).

So the final answer is that the backdoor 2 gap straight adds so little to your hand in a situation like this as to be almost worthless. (It does add some minimal value to your hand but not enough to effect flop decisions)