I would not be happy with a cm who was regularly mnetting. Sorry but I wouldn't. I know it can be a total time sink and I know at times it distracts me from my own children - and you know what, two wrongs don't make a right. My kids, my risk, my stupidity or whatever you want to call it, when I get sucked into reading something while thinking 'should really be getting on with X,Y,Z while the kids are asleep/playing happily alone'. If it was someone else's kids (whether or not I was being paid) - not a risk I'd take. If they were asleep, yes, fine, no problem with it from a safety point of view - although even then - it's such a time sink I'd still suspect it was taking time away that could be spent e.g. clearing up. As a bit of a net addict, I personally have a job sitting down for 20 minutes and not letting it slip into 2 hours - not a problem for anyone else's kids as I'm not a cm - but it wouldn't be good for anyone else's kids if I WAS a cm. As a parent I wouldn't be happy at an entire two-hour nap time being spent by a cm mnetting when some of that time (the time that's more than needed for their own relaxation/lunch hour) could have been spent by them doing clearing up, getting activities ready, etc. etc. (I don't care about observations though!).
I don't expect 100% of the cm's time to be spent watching my child, of course I don't - but I don't expect mumsnet to be one of the things they're regularly splitting their attention over during the day - it should be a lunch 'hour' (not whole of nap) or evening activity only IMO.
The bottom line is that whether or not anyone agrees, I would be happier with a cm who didn't know about mnet than one who did and might get caught up in an addictive thread and waste time that way. So a cm who mnetted would put me right off them - even if all it did was waste time (so then later when mindees were awake they were trying to multitask doing clearing up they could have done with they were asleep, for instance). I don't mean daytime mnetting HAS to be bad for mindees - but I can too easily see that it COULD be bad for me not to be put off someone who did it often.
Actually I'd be equally put off anyone I was paying to do something (e.g. normal employee) if I knew they were regulars on any board during the day, even if it was theoretically only for short visits - I know from bitter personal experience the potential for timewasting and distraction and it would put me right off. It's not about not wanting people to have a reasonable amount of relaxation time - even a generous amount would be fine by me - it's about the ability of mnet and similar to suck up far MORE than a reasonable amount of time that's the problem. I shouldn't do it as much as I myself do it as a parent - and I wouldn't expect anyone to do it a lot if they were my cm, either.