Gandalf could control the palantiri, cow Saruman, and raise himself from the dead. That's a little more impressive than dishing out some combat magic. Plus there's all that phat loot he had, like the magic ring that let him modify the defenses of Rivendell, and the badass magic sword, plus the skill he showed in wielding it.
I really cannot believe we're using LotR in an attempt to explain the 15MD. IIRC, the reason Gandalf didn't use magic much was because it drew attention, not because it was draining, or because he wanted to save it up for a "rainy hour" or anything like that. The real reason was to prevent him from overshadowing the other heroes, which shouldn't be an issue if you've got a party where all the PCs are roughly the same level. Book writing bears little resemblance to actual gaming.