Ah, yes. The wheelchairs...
I think they were intended to be a reference to an early-80's horror movie called The Changeling, as the series seems to love referencing horror movies and novels. Unlike The Changeling, though, SH4 really didn't establish why the wheelchairs were scary, and they instead came off as unimaginative or tacked on foes.
The only thing my milkshake brings to the yard is a subpoena.