this has been the case for me. I burned out on Skyrim two months after buying it last year (made 4 characters, clocked in 30+ hours on each), and starting up the DLCs has completely refreshed my desire to play. There's tons of content in each one, new spells, two new Oblivion planes, decent length main quest lines, new gear, spells, the list just goes on.
I'm not the biggest Skyrim fan in the world, but I've been playing a ton lately thanks to the expansions.
both expansions do in fact increase the functionality of the mage. the vampire lord added in dawnguard is a perfect fit for a mage, especially since if you take the vampire's side you can get some awesome rings which will help a mage immensely. there are several new spells mostly for summoning/healing/killing undead.
dragonborn is just full of stuff thats great for mages, everything from powerful new spells to powers and abilities that can make your mage ungodly strong. oh and staff making. you can make any staff you can find in the game assuming you know the spells and have the right materials... not to mention the new dragonpriest masks which can make destruction magic a bit more competitive. --- be more cereal!
Yes there all great dragonborn is the best IMO out of them all but still there all good like poster above said lots of stuff gets added there's also lots of other stuff added in the dlc even if u arnt a Mage like crossbows new weapons armor etc.. But if u do get dawnguard if u are a Mage def go vampire Warrior/assassin go dawnguard IMO