Things get a little unbalanced when you let dragons wear armor, though. And two simple buff spells - mage armor and shield - turn them from tough to nigh impenetrable.
In Red Hand of Doom we faced a dragon that was CR 13 when we were level 9. Normally this critter would have an AC of 26, which is respectable, but easily hittable by a powerful 9th level PC (our harpoonist, for instance, has a +17 attack bonus).
But the dragon had an ring of protection, and cast shield and mage armor, knocking its AC up to 35. Then it cast blur. Needing an 18 to hit a creature, and then having a 20% miss chance? Ridiculous. We would all have died, except an amazingly successful dispel magic stripped away all those spells.