Not "nothing important", but nothing at all. All my work contracts around here clearly stated that what you write in your worktime is the company's intellectual property.
It doesn't matter that you are not writing a thing.
Yes, that's possible, it still feels bad though (not to mention that most of the time sysadmins can still access my not exactly private machine) - why not work at a company that values me as a resource?
I can sit bored (with a couple of hours worth tracking in the timesheet software) for a month or two, but then I usually start looking for something interesting.
You're missing the point. If the rogue employee is already taking advantage, what's to stop them from doing so in a manner that also defrauds the company of any proof of ownership over a pet project?
You are sitting on your ass (conidering you are a dev) for eight friggin hours. If you include commuting, lunch break and the time you sleep, this is more time than you can spend with your family, hobbies, recreation etc.
You don't need to like or "couple your soul" to your job, it's part of your life and life shouldn't suck because there is a very good chance that there isn't another one.
If you have no scruples about getting free money, you can use the time to do whatever the heck you want. If you want to feel like you earned the money or you're not allowed to do anything outside of your scope, then yes, it's probably soul crushing.
If you do anything using company resources on company time, they own it. Anything you create during that time, the company owns. Do you really want to do something on the side and have the company legally own all the IP to it?
I was thinking about this - going down to a random employee and over a couple weeks trying to get them to let me help them out with their job. Learning over a couple weeks the practical principles of say 3d modelling, Marketing, etc. Sounds like something I would be down for. I guess at the end of it you'd end up being relatively popular in the office too if you occasionally help out where teams are struggling.