The Prometheus class has 15 decks like the Intrepid class, it's merely longer with a smaller crew compliment to support it's extra combat capabilities.
Deks: There is a reason why the Intrepid class is as powerful, which i mentioned before. It's pretty well known that the height of a warpcore generally creates more efficient power output. The Defiant class is able to achieve it's power by running the matter and antimatter up and down in loops to simulate a much higher warp core before reaching reaction. The Intrepid does it by utilizing the swirl design, lining the warp core with a dilithium lattice creating a 6 deck high warp reaction chamber yielding far more power than would otherwise be creating with a fixed focus reaction chamber.
Another thing as well, the only actual evidence I have seen that clearly says what type of ship it is was in the writers bible. There, it said the Intrepid class was a medium cruiser deep space exploration vessel. This makes sense when you compare it to the refit Constitution. The vessels are similar in size with the Intrepid actually having more internal volume. The neck in the Constitution was pretty much useless so if you take that out it eliminates something like 4 or 6 decks, making it a total of around 15 decks like the Intrepid class.
The reason why it has more internal volume versus the Constitution class is because it's primary and secondary hull are larger. The Primary hull on the Constituion refit was a concave design, which limits the amount of interior room, and wasn't as long as the Intrepid class. Wider, sure, but not as generally long or as big. The Intrepid class' primary hull was convex, yielding more interior room in a longer and generally larger package.
That said, the Constition was a heavy cruiser/battleship, so taking the size comparison in to account, it makes 100% sense that the Intrepid class is a medium cruiser/deep space exploration vessel, with the combat capabilities of a Galaxy class starship, and with some refitting even a Sovereign class if you take into acount the Prometheus class which has the same deck height (though this would be pointless with the advent of the Prometheus class.)
I agree with you Deks that they wouldn't entirely suspend the Galaxy class. I just think that because of it's original design purpose it's a WAY more flexible design than a Sovereign. And I still think the Sovereign is pointless when you have the Galaxy class, Intrepid class, Defiant class, Akira class, and Prometheus class as the premeir classes of Starfleet (those classes of ship are pretty much support all the major roles the Federation needs them to fill.) Making the Sovereign class seems superfluous and wasteful, and probably more expensive in the long run, and definately contridictory to Starfleets peaceful nature(the thing is a DEFINATE Heavy cruiser/battleship, with scientific applications on the side.) I know the Defiant is also a war ship and the Prometheus is too, but they're way less imposing and thus more compliant to Starfleet's peaceful nature. the Sovereign class just looks MEAN. But that part of my argument is just opinion, I still stand by the superfluous argument to the Sovereign class.