I hear what you're saying, but it would interrupt ongoing storylines in established indies feds. Besides, you have a good number of indies wrestlers (like Bryan Danielson until very recently) who had been in the indies scene for ages and were doing just fine there.
Take Sara Del Ray for example. If she were to get pulled away from Chikara right now to be thrown into the TNA Knockout scene for a possible 7 minute match against Lacey Von Erick in a match nobody would care about, it would really hurt my favorite wrestling angle at the moment(let alone the storylines she has in Shimmer).
Although I read an interview about Sara Del Ray being approached by TNA a couple of times, she ultimately got rejected for not being girly enough (which is odd since she's the ultimate PAWG wrestler. Much bigger ass and legs than Thicky Mickey, and a much better wrestler).
I get what you're saying but I don't know if that many Indy wrestlers are established to the point that they can make a good living just doing the Indys. Now the other thing is that TNA isn't paying a huge amount of money(its been reported that the average salary is $200-400 per appearance to female wrestlers) so it would be strictly for exposure.
Besides Del Ray is an average looking woman when she doesn't have the crazy makeup on but it seems as though TNA doesn't care about unique wrestlers preferring to try to go the WWE Diva route as this point.They have some solid workers but it seems as if they have made physical beauty a priority.