You're correct that it's not recommended and can possibly hurt your anonymity and privacy but it's important to understand the reason so you can make the right decision for yourself.

When you install add-ons in tor browser (besides NoScript which is installed by default) they narrow your specific browser fingerprint. JavaScript enabled sites can get a list of browser add-ons you use. But many people still install uBlock origin on tor browser. As long as you don't install 5 add-ons it won't hurt your anonymity much. Installing uBlock Origin may narrow your unique fingerprint down to tens of thousands of people instead of hundreds of thousands or millions, but it won't make tor instantly worthless and can in some cases improve your security.

It all depends on your risk assessment. Are you just looking for extra privacy and anonymity, or is there a large target on your back put there by a government agency or organization of some sort?

https://browserleaks.com/