Does Blender bloat unnecessarily the Windows Registry? According to Revo Uninstaller, Blender adds more than 4400 registry keys to the Windows Registry. They are almost all spread in Registry folder:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Installer\UserData\[your user string]\Components


Assuming that the Windows Registry is kind of a highway (database) where system and user applications will eventually want to quickly store and share data, unnecessarily growing it will impact other applications performance on accessing, not to mention the impact on registry backups that Windows makes periodically.

Next follows an old opinion of Jeff Atwood about using registry as a storage room (source: here): 


  a trash heap of miscellaneous junk settings for every rinky-dink application on the planet


Many of the 4400+ entries that Blender stores in Registry, if not all, are references to Python scripts, that I strongly doubt they really need to be stored in a system wide database, instead of a local configuration folder.
If you are unsure about entrys in the registry and want to keep your system clean, you can take the portable version.
Extract to a folder of your choice and pin to startmenu or what ever.

I personally dont use Blender installer at all, i use the portable every time and when there is a new build i override the folder content and have the new version.
If you have multiple versions you can have different folders.