Sizing by seattube (conventional size labeling) IS an issue for people who are buying frames over the internet and cannot test-ride them. A high BB can make a company's declared 57cm bike fit like a 59 or 60cm square bike.

mihlbach knows this. All he is saying is that sizing conventions vary widely across companies, and at best, they should be treated as rough guidelines only. Ditto with top-tube to ground measurements (standover height). Consumers should never trust these numbers, but instead look at the actual stated geometry and length of the tubes--particularly the TT length, I'd say--when deciding which given size of bike to buy.

seatube/standover sizing and BB height is also an issue and a source of confusion in cyclocross bikes. Standover actually does matter "more" in the case of 'cross, given its off-road nature. But mihlbach is correct that in road applications, you can have very snug standover heights without it being that much of an issue. I remember reading an 80s bike book that said if you could straddle the top tube while leaning the bike over, you were fine, although I think that's pushing it a bit.