Depends on what you use your Pi for.
If you use it to develop, like I do, and for that use git or some other distributed version control system, it gets pretty annoying to have a new commit dated 3 days ago.
If you need public-key infrastructure certificates, that have begin and end times on them, you will have expired or not-yet-valid errors at some point.
If you run a backup application, that needs to schedule when to suck data from your network and store it elsewhere, you will need accurate time to make sure you won't do that right in the middle of the day, where open files are everywhere and network bandwidth might be affected.
But yes, for all those reasons NTP isn't necessary. You can setup a rdate or ntpdate on boot and not have ntp trying to compensate for clock fluctuations of your processor, at you will do just fine.
But there is at least one use that I can think of that will need NTP-level accuracy: parallel and distributed use and processing. Be it a load balanced web server, be it a file storage system, be it a file mirroring task.
For most of the other cases, you can just replace NTP with something else, like I suggested earlier, rdate or even ntpdate. The latter seems to be easier to get going these days, given that NTP is pretty standard and rdate protocol ports are almost certainly filtered or closed everywhere around.
I have had one Pi running unsynced for some time, on a air gap security application. The experience was awful, even if the application did not need any sort of time-accuracy. I would get crazy trying to figure out the creation or change dates of files, trying to figure out information from logs (when did that happen?!) and all sorts of things that you usually place in a window of time to understand or analyse.