Spellcasting dragons bore me. I like dragons being fierce and strong, but I don't like them being less-diverse spellcasting fiends.
If you tick off Smaug he's not going to cast a spell to find you, teleport into your bedroom and eat you. No, he's going to burn your town down. If you tick off a fiend, he will probably corrupt all your family and friends, destroy your reputation and sanity and then torture your soul for eternity.
This doesn't mean individual dragons can't be spellcasters, of course, if that's part of a specific dragon's personality or a specific type of dragon's powers. Does a red dragon really need to be able to cast Lightning Bolts or Mage Armor or Heal? She's already a huge engine of destruction with six attacks per round, incredibly hard scales and the ability to destroy armies just by breathing hard.
Smaug is cool, but Smaug is not a D&D Dragon.
D&D Dragons are awesome (at least in many of the novels and in some of the campaigns that I have been in) because they often are behind the scenes manipulating stuff.
When they do come out or when adventurers come knocking, not only are they engines of destruction, but PCs have this annoying habit of killing everything. So as reoccurring villains, non-spell casting Dragons tend to bite the dust.
Spell casting Dragons, on the other hand, scry, and magically protect their lairs, and when they are losing a battle, they Dimension Door out of immediate melee and fly away. As the range attackers try to bring them down while escaping, a Shield spell comes up and protects them from a lot of that.
They are not just engines of destruction, they are reoccurring villain engines of destruction. Even when the PCs start out leveling them, they are still there and can still kick butt in a fight.
One of my most memorable games had two dragons attacking the PCs at the same time in 4E about 5 years ago. Normally, this would have been an average encounter because the dragons were not that high level. Instead, one dragon had a cleric template and could do all kinds of buffs and heals. Most of the time in 4E, long fights were grindy. This one was not. The players were pulling out all of the stops (every Daily power, every item power), trying to figure out ways to end it. After about maybe 12 or 15 rounds, the dragons flew away, bloodied but not killed.
To this day, when I talk to some of those players (who live 2000 miles away and we don't game together anymore), they once in a blue moon bring up that dragon fight. In fact, I showed the first post in this thread to my daughter as I was writing it and she said "Yeah. Remember those two dragons...".
Spell casting dragons, IME, tend to be more memorable and exciting than something with natural weapons and a breath weapon. One is versatile, the other is just another tough monster. IMO.
I often come back to the concept of, this is Dungeons and Dragons. I want there to be a lot of cool dungeons and I want there to be a lot of cool dragons.

I'm hoping Tyranny of Dragons will fall right into that wheelhouse. Dungeons, and Dragons.