Theoretical question about heads up

[cheburashka]

Maybe this is a beginner's question, so forgive me if I've mis-posted, but I find that you guys generally give great advice, so I'm posting here.

Harrington sez SB should will always have the correct pot odds to call, because you are getting at least 3:1 (depending on antes), and even 32os is better than a 3:1 dog against the average hand.

But something strikes me about fishy about that.

If you are calling all-in, then the pot and hand odds obviously apply. But in most instances, you are not calling all-in, so if you don't pair something on the flop then you are extremely vulnerable to betting. If BB is any kind of aggro, then there is a 90% chance he bets the flop, and then you have to fold without seeing the turn or river.

So it seems to me that for at least the worst trash (i.e. hands that are very likely to meet two or three overcards on the flop), the relevant odds are not your hand vs. the average hand, but the odds against pairing the flop.

I've done a search and can't find a good answer to this. Can you all help me out?