I was kind of expecting someone to tell me "Spacer" was too obvious and to obfuscate it some more, but they let it slide - probably because this is not a security flaw like I'd usually be dropping, just a general oopsie.
More importantly, if they're installed on a large number of machines, they become an easy target for malware authors - observe the number of exploits targeting vulnerable link handlers like steam and uplay's, where it was possible to invoke an arbitrary executable from a hyperlink.
The most common (at least based on my ~10 instance) technique is malware that installs itself into %APPDATA% and sets itself to start on boot. The executable then launches some process (like services.exe) and injects its own code (known as RunPE).
I'm not sure how prolific exploitation of user-mode binaries is, but the damage that can be done from user-mode is non-trivial.