1) I dont think my car is in any way in need of traction control. Its nice in the snow I guess on my other car. i never have it on. I dont like what it makes the car do in the event that it it does lose traction.
In my other car tcs or asr or whatever it is called has almost cased me to get hit on more then one occation. I try to pull out of a parking lot or something when it is a little wet or something, I get a little spin the car applies brakes and rejects throttle making it so I can not get up to speed of traffic. If disabeling it didnt cost money I would no longer have it.
I could see it being useful on a really high performance track car with a much more sophisticated tcs system for getting through turns with out spinning. But imo has no place in my street car. Id rather have it spin and have the car do something that I can correct then just stop or react differently then it should to my imput.
2) No shouldnt decrease fuel mileage. Should make the trany last longer by lowering how much it slips on shifts. As has been said fuel mileage is way more effected by your right foot.