Remember though it was a Klingon Engineer who discovered how to defeat the energy dampening weapon of the Breen. Without it the Breen are on an equal footing with the Empire now. The empire never revealed the secret to it's allies either.
Actually the Klingon engineer came upon this by accident when making a adjustment to his ships warp core that was having containment problems.
Because the feds and Romulans use vastly different tech to the klingons they could not make the same adjustment to their ships and a different counter had to be found. http://memory-alpha.org/en/wiki/Energy_dissipator
The "I want a pretty/sexy toon" isn't limited to Orion and alien girls, it's not even limited to STO. It happens in every game, everywhere. The funny thing is that, according to surveys, Klingons are voted as the species with the most attractive males, Vulcans the species with the most attractive females. (I guess the latter is the doing of T'Pol alone, lol) Maybe I just have a weird taste, but I think some of the most attractive females in Star Trek were Klingons (Grilka, B'Ellana) and that's why I don't really get all the supposed-to-be-sexy aliens.
I don't think any of the species suggested in this topic would change the numbers of other species females. Breen would very likely end up like Gorn, Letheans and Nausicaans - without a female option. And they appear to be the 'most wanted' playable non-Fed species so far. I have a feeling the real solution to get more people to play Klingons is... better clothing options for females. A dress or corset like the Duras sisters usually wear - the "hello, may I introduce my cleavage" dress. Be sure that option and the excessive use of the boobs slider increases the Klingon population about 50 %.