Well he can always summon them if the situation needs it. Gamakichi must be enough for this situation, and it's good to make a diversity in which summons you use, since all may have different abilities, and thus it improves your own diversity for different situations. Jiraya did the same in most of his fights.
I think it might be another of Kishi's themes at work. The next gen surpasses the old one. Looks like Sasuke and Sakura have new summons as well. So it would make sense, because of the set up for Team 7 to replace their masters.