I guess it just depends on what you want to do. Most guys here go for Jiggs or Tifton 85 bermuda. Anybody you talk to, they act like that is the only grass in the world. You ask university types and they'll try to talk you out of natives everytime (around here). Guys love the bermudas and other improved grasses because of the tonnage and protien levels, and digestibility, TDN, etc. They also love it because it looks pretty. Problem is the tons of fertilizer you have to put on it to get those yields and protien. And fertilizer ain't exactly cheap anymore.
Natives, on the other hand, save you all that fertilizer. I saw on a seed website lately that bluestem will produce it's maximum potential on 50 lb/acre of N a year. Sure beats 300+ top yields of bermuda. BUT, the protien on natives is probably half to a third what it is of fertilized improved grass. TDN, digestibility, etc are all usually lower with natives too.
Seems to me that lots of folks focus only on the numbers nowdays. It's like,"WOW! Look how much that improved grass is going to make compared to natives!" or "Man look at the protien/ TDN/ etc. levels of that grass compared to the old standby", etc. There is more to it than that. Cost of production, for one thing. If you can produce a field of native grass using clover to make your nitrogen for you during the fall and spring, you sure will have more money in your pocket than spending it all on N-P-K. Even when you factor less protien, less digestibility, etc. into the equation. Aside from the numbers, like somebody said, management is a big deal too. You can't graze natives into the ground and watch em spring back up like improved grasses. But you'd be surprised what a native mixed stand can do.
I've got 14 acres of mixed native with some dallisgrass and some bahia (I hate bahia) invading. It has never seen the plow. It has never seen a fertilizer cart either. In a wet year, I have baled as many as 85 5x6 rounds off of it. WITH NO FERTILIZER! It doesn't even have clover on it! I told the NRCS guys about it and they were just floored! I wish I had the other 65 acres of the place just like it, but they were in cotton for the last 100 years. I also told them how my Great-Grandad used to bale hay on the same field 100 years ago and it's never been fertilized to my knowledge. He sold hay to the Army in WW I for $1 a bale and got rich.
So, depending on what you want to do, everything has it's place. If you want 8 ton bermuda with 25% protien, etc. you're gonna pay the fertilizer guy to get it. But if you're selling enough hay, feeding enough stockers or whatever to pencil out, great! If you can live with 6 ton bluestem with 8% protien etc. and can run a little "understocked" (by neighbor's thinking, anyway) you can sure get by a lot cheaper with natives. Personally, I plan to use a little of both. Diversity is the best idea. But with costs like they are for NPK, like grandpa used to say, "it's not what you make but what you save". Course he also said, "you gotta spend money to make money" too. Guess a bit of both is good. Good luck! JR