There is no rule about not being able to use old models. The only thing I can remember is that you have to use the base provided with said models.
That being said, every Tau player I know refuses to upgrade to the new Broadsides because it his harder to hide the bigger models in cover, or put them in ruins. Honestly I think the old Broadsides look like crap, but I will never get to see a new one fielded because all the Tau players I know don't want to pay money to "nerf" their Broadsides.
With way the people I know talk about it, I do feel they are "modeling" for an advantage. But there is nothing I can do. Personally I would prefer that everyone always used the newer models, no matter their size, I know I do. When the new Necron Wraiths came out I upgraded all of mine, despite the fact that they are nearly twice as tall as they where before.
Peace, Love, and Mass Destruction
Necrons in 6th Edition W: 28 L: 9 D: 5
Orks in 6th Edition W:9 L:9 D:3
F*** that, Barrel rolling monolith all the way.