Because flying boots cost 7 gems with hammer, and they can be passed between commanders as the need arises. Summoned mage, on the other hand, is just that - a flying S2A2 mage who cannot cast anything your inexpensive national mages cant.
But 16 astral pearls is surely better than 25. At least now I would consider summoning them in a besieged castle, for example, to reinforce communion. Perhaps... some other uses as well.
Actually, I was just aggroed by a suggestion to rush Conjuration tree to start summoning Gamayuns.
They may be considered a bonus, an addition to something else, but you just cant base any strategy around them.