And to think he didn't see her topless yet (since her rack is one of her most notable aspects)

I'm not understanding the usage of ありがたみ(有り難味). Is it some kind of comment on her out of character / indecent behavior? 　Or a remark that this form of exposure is unsatisfying?

It is a bit cryptic. I take it as something like［そうすれば］ありがたみが［ない］, indicating that seeing her panties loses its appeal if she just shows them openly.

I got that impression as well. So it's something like 'But that doesn't make me happy...' or more along with your note it would be 'But then it's pointless...'