It would seem to me that having at least one in your book would be worth it. It's one more mana than a grunt, and the attack isn't that much worse. If you are spamming grunts, he'll do almost as well. Further, if you are boosting your grunts, your boosts will help him as well. To me, he's worth the inclusion for those rare times you might need him. If you've got excess mana, or if you don't want/or can't venture your mage to an area of the board, he can go and cast those caltrops, walls, or garrisons. If you don't need him for, he's a card that stays in your spellbook or gets used in a pinch if you need more weenies. 1 extra mana for a cast and 1 less dice on attack seem worth his potential utility.
would I include more than one... doubtful. As others have said, multiple builders only seem good for their healing ability, which isn't that good.