[cougar62]

First question: basically the bigger your stack and EV the bigger your edges need to be. To play those hands you need to know that the implied odds are enough to justify getting involved, in other words, is there a good chance you're going to stack someone if you hit? Otherwise your EV doesn't improve that much by taking shots and hitting these speculative hands.

That pretty much answers your second question too. You're giving up your EV edge when you don't hit, and a lot of times not improving it a hell of a lot when you do.