Same as yourself OP, when I built a new PC I would be testing things out and installing programs and utilities etc. Once testing was done and I knew what I wanted to run and the tweaks I would put in place I would do a fresh install and then keep things clean and only install what I was using.
Now I tend to build it and get it sorted, fresh install and then with SSD's etc like others have said, you don't really notice the system bog down as much. With SSD's the only time I rebuilt in the last 18 months was due to moving to windows 10 and reinstalling Windows once the licence was swapped over from Win 8.1.
Other than that, if I swap motherboards/cpu or a HD fails then I will always do a clean build, saves faffing with driver issues etc
When I was younger and in uni etc I had time to waste on spending days tweaking everything, now I get it up and running and then watch what I am installing on the systems.
Use the laptop for pretty much everything now and then the gaming PC has the bare essentials on it and then the games. Laptop is imaged so if I kill it, takes 20 minutes to get the image back on