I've noticed this happening more and more often over the last few months. And in some ways, I do imagine it's much easier. You don't have to rip/clip anything yourself and you're working with smaller files/file sizes which probably helps with editing/rendering. But it's also pretty limiting and the quality can be very mixed. I imagine if you were doing it for a large fandom with lots and lots of gifs of good quality to choose from, it'd probably be a good idea. But trying to do it with a limited amount of poor quality gifs...well, I couldn't do it, that's for sure.

And even though I don't think it's quite the same as stealing someone else's vid and using it to make your own, it's still not on the up-and-up, I don't think. I'd probably ask the original gif maker if I could use their gifs before I'd ever consider it.