Thanks Hrishikesh for linking accuracy of Naruto running with actual Ninjas. (As an aside, I'll point out that no marathoner runs like ninjas so I'm skeptical about its efficacy wrt running itself.)

You are correct that the number of _frames_ needed to render many running is the same though do note that _drawing_ more complicated running is an expensive affair. It isn't as straightforward as just drawing running. The artist must ensure that the characters on screen are not in perfect sync with one another — without this care it'll appear as if the characters are marching. By keeping their arms rigid, the artists have neatly side-stepped this problem thereby saving costs.