Agreeing with what else has been said and would like to add, this punter probably won't realise what it's really like, What about when he gets a weird, icky, slightly smelly client that you could probably handle and wants to run away?! You can't let anyone else ruin your professional reputation.
Also do you want a punter knowing your tricks of the trade and maybe realising that sometimes you aren't really enjoying it or are faking it.
I got my partner in for a couple of MMF's (I wasn't paying him!) and ended up sacking him because he would get obviously bored mid booking! I think it REALLY made him appreciate my job though and it made me appreciate my own professionalism.
If you were a dentist you wouldn't let a random man help help out with a filling- I think we have to look at our own jobs with the professionalism other people do.