I've done this a fair bit since I bought Rahzel's album MTM 2000 or Make The Music 2000. Rahzel is the biggest human beat box around these days. I know of an old school human beat box called Darren Robinson but I haven't heard his shit, so I don't know if he used to scratch vocally.
It's really not that strange of a habit as far as I know, considering a few recent top 100 examples:
Slim Shady's vocal turntable - "Hi my name is tKEtKE Slim Shady"; and
Afroman's Because I Got High - "buk bukka buka BUKAW!" (slightly amusing how the last one sounds like a chicken)
Some things I always find it easy to scratch are anything starting with 'R' (probably because I learnt from going "Rah-re-Rah-ra-re-re-Rahzel") and my rap name Chur-Ch-Chur-ch-chka-chka-Churd-Tzu.
Last time I was jamming and DJ Freezone wouldn't let me use his turntables so I used the mic instead. Everyone thought it was kind of cool, but the ones who appreciated it most were stoned.