Pot odds Preflop in NL Hold'em

[Nacarno]

It seems to me that in books detailing plays preflop in NL Hold'em, they often recommend calling if you're getting pot odds over 2-1 heads up because you can't be that much of a dog to anything but a high pair. For example in HOH Vol. 1 problem 4-1, Harrington recommends calling an UTG raise from the BB with 76o because you're getting 3.5-1 on your call and "even if [your opponent] is holding ace-king, you're only about a 2-1 underdog at this point." Now, correct me if I'm wrong but isn't 2-1 the showdown odds against AK? What about that little something called a flop?

If you flop a pair, there's a good chance there will still be overcards out and you need to be weary of them. Chances are very low that you'll flop a straight and your draws are most likely to be charged. You can only really be happy with two pair (and that's still dangerous with lower cards) or trips. Now I'm not certain of the calculations, but it would seem that you're a bigger dog than 2-1 to hit a favorable flop (as described above) and you're likely to face more action and pressure from the UTG preflop raiser. True, implied odds may make up for some of this, but it general it seems like if you're playing against strong players, you're putting yourself in very dangerous situations. Of course, if you're an even stronger player yourself (and you'd have to be since you're out of position too), you may be able to overcome this, but I still feel like it's kinda dangerous advice to be giving most players reading these books.

Is there something I'm missing that makes this advice more correct and clear-cut? Also, is anyone aware of an odds calculator that would tell you your chances of having the best hand just on the flop? That might be more relevant to pot odds before the flop.