Not all instances of reusing footage are bad. In the same breath, not all of them are good either. It can depend on the shot being borrowed, but a general rule of thumb should be the less iconic, the better so that it doesn't stick out like a sore thumb the next time the audience sees it.
Now right away, there's an entry on this list that breaks that rule, but overall, taking the big and bombastic moments to retool them isn't the best idea. It's simple - if something is so good that it is instantly burned into your memory, then using it again runs the risk of watering it down. Sometimes, reusing footage can lead to confusing the audience.
Wait, isn't that character supposed to be somewhere else entirely? What did we miss here?
Having said all of that, the smaller moments, those that may not stand out as much, can be borrowed, copied, and pasted heavily. It saves on budget, it saves on time, and it can even go a way toward raising the overall quality of the new episode. There are a few examples here where, were it not for graciously swiping some earlier shots, there may not have been much of an episode at all.
10. Excelsior Model - Star Trek: The Next Generation
ILM was contracted to do the visual effects work on Encounter At Farpoint, the pilot episode of Star Trek: The Next Generation. ILM had of course worked on several of the Star Trek films, so they were well used to the various props and sets. In fact, they were particularly used to the studio model for the USS Excelsior, having filmed it back during Star Trek III.
The script for Farpoint called for the inclusion of the USS Hood, so ILM opted to take new shots of the model. They then set about capturing as many stock shots as possible, for including in later episodes.
This footage was then reused many times throughout The Next Generation, including Best Of Both Worlds and Chain Of Command. The Child, the second-season opener, used some of this footage as well, with a twist. The shots included in that episode hadn't actually made it to air in Farpoint, which meant that in that episode, it was their first appearance - making it both a reuse and a debut. This footage would remain in use until the digital models replaced them in Deep Space Nine.