When we bought our new coach, a tag axle was a must have. Typically, you don't see a tag on anything under 40'. At 40', without a tag, you get a great ride (think limousine length). You also get to a point where 40', without a tag, gets a little heavy on the rear axle and you limit the CCC. So at the 40' length you need to decide how much storage you'll need. Anything over 40' needs a tag and will really add to the comfort and handling of the coach.
As I said, we wanted at tag, but we also wanted to stay at 40' for several reasons. Our coach handles great and rides nice, but our storage is not much bigger than our 36' DP that we sold. The tag axle takes up three of the extra four feet we added. We also have a side radiator and DEF, so we lost all storage behind the rear wheels. Once you get into the 42' and larger you start gaining all that extra space back.
One of the things I didn't realize with the tag on a 40', is I can now turn sharper than my 36' could. Technically, my wheelbase from front wheel to drive wheels is the same as the 36' and my front end has a 60 degree wheel cut.
I agree that turning, lifting and adjustable pressure tags are the cats meow, but you're only going to find them on really high end coaches. Most of your new coaches have a tag that lifts, but not off of the ground. They lift just enough to take the pressure off and keep from tearing up the tires.