✔ 1 accepted answer: ALWAYS a resounding NO!
Like most "accepted" answers online, this one is wrong too.
Piggybacking chips when troubleshooting boards been been done for MANY years, and is a perfectly fine way to troubleshoot, as long as you understand that it's not guaranteed to pinpoint your problem, and in some cases could damage the donor chip that you're piggybacking (if the original chip is bad with a stuck output, and the new chip can't handle the current from the conflicting output).
Think about the cases... in the case that you suspect a bad chip, but it's not actually bad, the chip that you piggyback and the original one will output the exact same data, and nothing will happen. In the case that you piggyback a good chip over a bad one with a weak/open output, it'll fix the problem. In the case that you piggyback a good chip over one with a shorted output, they will conflict, and may meet in the middle, or one might overdrive the other. In some cases this will look like nothing happened, or it might drive it enough to make it marginal or even look fixed. Even subtle clues like that are very welcome when troubleshooting boards.
A logic comparator is nice as well, since it does actually tell you that the logic differs... though they're not perfect either. Plus, piggybacking is possible on chips where the logic comparator doesn't work (analog chips, RAM with multiple voltages, etc).
So, like most things... it's one tool in your toolbox. As long as you understand what it can/can't do, and you know the risks... feel free to use it when appropriate.
Obviously when you do determine what the problem chip is, properly remove and replace the bad chip... don't just solder the new one over the top of the bad one.
DogP