This does not 'install a backdoor that allows any website to take over your computer', right? It just makes it possible to launch any previously installed executable if you know the path.
Well yes, it allows "offline" privileges to essentially any online site (if you can launch arbitrary executables, you can download and execute arbitrary payloads). And considering there is still a rather prevalent culture of running Windows as an administrator account (if only because some softs fail rather annoyingly and without trying to escalate when launched without adminstrator priviledges) for all intents and purposes it gives pretty wide control of the machine to any URL you connect to.
You say that as though it's some kind of hurdle.
C:\>ftp -h
Transfers files to and from a computer running an FTP server service (sometimes called a daemon). Ftp can be used interactively.
FTP [-v] [-d] [-i] [-n] [-g] [-s:filename] [-a] [-w:windowsize] [-A] [host]
I am in no way trying to say that this can not be dangerous, but it's different from what we would usually call rootkits.
For instance, the remove-item commandlet, its description goes like this "The Remove-Item cmdlet does exactly what the name implies: it enables you to get rid of things once and for all. Tired of the file C:\Scripts\Test.txt? Then delete it"[1]. No UAC prompt. Bingo, let's start erasing this annoying C:\Users\Username\Documents.
And this is only one example, give me 1 hour and I can find several ways to fuck up your computer with a powershell open :-).