I’ve said this a couple of times, but because it suits my prejudices I think it deserves its own post.
Recycling material is fine so long as the material is good. And often it is good, especially when it’s a more formalised, expected component like a transformation or combination or launch sequence. I don’t mind watching goodness several times. In my better moments, I rather like the idea of watching goodness several times.
Furthermore, stock footage has a certain reliability. However dull the rest of the episode is, at least I can bank on the manly combination to be great. This is an efficient way for animators to spend money and time.
And if you’re not convinced that repetition has its place, consider the two most commonly recycled elements of your standard anime episode, which we all take for granted: the OP and the ED.
(Would I take much the same stance on grander kinds of re-use, like repeatedly using the same premise? Of course I would. Of course.)