I see your point. Of course the villain could pot the turn when you raise flop. But if you can assume that you have 15 outs you can call a pot sized bet. 15 outs gives you about 1:2 to make your hand on the next card which is the pot odds you getting against a pot sized bet.

If the villain has a set or two pair then you may not have 15 outs, but then you may not want to be all in on the flop either.