And if you find a bug? If it's in a recent nightly or a stable release, you can rightfully file a bug. It will be either fixed on master or a point release will be made. If your nightly is weeks old, you don't really know whether master has fixed it weeks ago or a bug report would be welcome. So you lose this advantage.
Anyways, that's not an exhaustive list. Not wanting to deal with rustup might be another reason so you use your distro rustc packages instead.
Finding a compiler bug is rare... but when I do being on an old nightly tends to speed up the reporting process. That's because this is the process
1. Search the bug tracker for the message/symptoms. If you find the bug (more likely on older nightly's) stop here.
2. Create a minimal test case by creating a copy of the code and then deleting large portions of it until you have a dozen or so lines and no dependencies that reproduce it and are all needed to reproduce it.
2.5. If using an old version of rustc, `rustup override set nightly` to use a recent nightly on this directory and see if the bug is gone/changed. This takes about 30 seconds (compilation is instantaneous since I've already deleted all but a dozen lines of code).
3. Search the bug tracker again now that we have a better idea what is going on.
4. If there really is nothing, report it.
The time savings by bailing out early at 1. greatly exceed the time cost introduced by 2.5.
If you're using nightly you should be using rustup... but since rustup is so easy to use that's a very very low cost.
Anyways, that's not an exhaustive list. Not wanting to deal with rustup might be another reason so you use your distro rustc packages instead.