My mom and I used to play video game Wheel of Fortune a lot when I was a kid. We came to the conclusion that buying vowels was generally bad strategy, because of how easy it normally is to determine the vowels in a word given enough consonants. (It is generally much easier to fill in the vowels, having the consonants, than the other way around). If you are confident enough a particular vowel's usefulness in filling in a word to buy it, why not keep the information to yourself instead of revealing it right there on the board (at a cost to you, too)? You may be giving yourself an advantage if another contestant hasn't figured that word out yet.
Of course, in situations where the word could plausibly be "in" or "on", you have to bite the bullet and buy to determine which it is. But that doesn't seem to come up in too many puzzles, and one of your competitors will probably save you the money and buy it out for you, since they won't know what it is either.