Ah, I think I see the differences.
Like any program that you install on windows it manages to do a crappy job every time of cleaning up. When you hit that uninstall button it's more or less an illusion making you believe it's uninstalled but in fact it's left registry entries and files and folders thrown in every corner of the drive. I'm sure you're still wondering okay why is that a problem? Well the problem is not very obvious until you actually have a problem. .
.. Let's not even try to get into that scenario. What would be ironically even more painful if it was the same scenario, and you didn't actually have a problem you just wanted to upgrade to the latest version of visual studio. Well in that case good luck.
...Instead what I see more of is problems arising from just too much crud building on the hard drive. Hell just reading tech support forums. In which most of the time their problems come from the amount of different programs installed. Also programs that were uninstalled but never cleaned out properly, etc.
... Apparently from the responses no one has ever attempted to uninstall visual studio. I encourage you to setup a virtual machine. Install visual studio on it. Then attempt to uninstall it. When the situation get's to the point of wondering if wiping the hard drive would be a better option. You will understand my reluctance. It honestly act's like a virus. Which was the whole reason I made this thread.
First, you absolutely can install multiple versions of visual studio. I have 4 versions installed on this machine right now, and frequently switch between them as needed. No uninstall is necessary.
Second, it looks as though you are talking about very long term situations. Right now long term stats for HDDs suggest a median lifetime of about six years, a bit longer for casual home users. If you have an installation of Windows that predates 2008 you should seriously consider replacing it and starting over. At four years old I'm already starting to distrust my current installation. Sure you can trust your luck and keep using the machine until you face a hardware failure, and then pray to the silicon gods that all your data is recoverable, that your backups work, and that you won't need to pay tens of thousands to a professional data recovery service... or you can replace your equipment at recommended timetables.
Visual Studio is not a typical program. It isn't like the tiny little applications that can live as a single executable. It isn't like somewhat larger apps such as Word or PowerPoint or a web browser that only needs a few contact points with the system. It isn't even like a device driver that can interact with the system in protected ways but usually sits quietly. Visual Studio is a programming environment that has tendrils all the way down into the operating system's kernel; it provides and replaces device drivers, and it includes a long list of programs that range from little self-contained executables, to OS system services, to potentially high demand server components, to deeply integrated debugging hooks.
The VS team has done quite a lot with the uninstaller. If you install visual studio and then immediately run the uninstaller your system will be basically intact. But installing and using Visual Studio is highly invasive to Windows. If you are familiar with Star Trek, it is much like the conversion to becoming a Borg; it impacts almost everything and after a very short time a complete reversal of the procedure is impossible.
A software development workstation is very different from a normal consumer's installation. This is true of many serious computer uses, such as data center computers and superpercomputer nodes. While you can run much of the same consumer software on it, that doesn't make it a good idea. I can install Steam and run all the latest games on a data center node, but I would be expected to completely wipe the machine rather than just uninstall the software when done.