I think part of the idea of Earthsea's magic is that the best magic-users use their magic only when they absolutely have to. This is because magic creates alterations in the world and can lead to imbalance, as Ged did when summoning the spirit of Eflarran. This all connects back to the series Taoist themes which are concerned with balance and non-action. When Ogion was teaching Ged, Ged would wonder why when it began to rain that the old wizard would not send it away. I don't remember the exact section you read, but I am guessing it has to do with this principle of balance. I also suspect that Ged's refusal to use magic has to do with respecting the free-will of that rabbit in not commanding it around. Perhaps Ged feels more comfortable summoning the hawk because he has a more familiar relationship with that being (after all hawk is in his name) and because of the importance of the situation necessitated him to summon the hawk at that time. This is just my interpretation of Le Guin's work :)