Well firstly as someone who owned and played NBA2k5 and 2k6 extensively, I can tell you they are nowhere near being "essentially the same game." There is almost NOTHING in common between the two.
The MLB/WSB series has a great track record, albeit one that I don't agree with the majority on. Madden 06 was obviously rushed for the reasons you stated, but can't you see that MLB2k5 was similairly rushed? There was no new console but there was certainly the need to put the product out there before in a timely manner with regard to the competition. I'm no MLB diehard by any means, in fact I haven't played a baseball game I consider to be all-around quality, maybe ever? They generally don't get the same love as basketball and football games for whatever reason.
That said, NBA 2k6 was enough to convince me that the next baseball game might be worth at least playing.
i should confess that i am no basketball expert or purist, but i know the sport well enough. i played in GAF leagues online for nba 2k5 and nba 2k6. there isn't much difference between the two games, and yet you say they have almost NOTHING in common? how about the following that they do have in common:
1. no true defensive stance button
2. "run around with any speedy player for 20 seconds and then go in for a layup"
3. no interior defense
4. "dunk through a player" animations (i.e. dude goes up for a dunk, there is a guy there to block it, but the dunk animation still happens without consequence)
5. no seperate pause menu's online (how hard is this VC??)
6. no "divisions" for online play
7. no incentive for mid-range game rendering many top notch players useless (elton brand anyone? i know from experience!)
8. ridiculous AI "subbing" logic
i'm not saying nba 2k6 didn't improve, but the game was still pretty broken on 360 and xbox. you are free to disagree with me and i'm sure you have a laundry list of things that were changed. i should also note that i dont play this, nor any other sports game, for its franchise/season modes. therefore, sliders cannot be taken into consideration (even though VC had said they would allow online sliders in league play, which was a lie).
about mlb being rushed, couldn't the same have been said about MVP baseball, by far the greatest baseball game of our lives? they were "under the gun" to get the game out as well. it's not like VC was working with new hardware or anything.