Just as there is plenty of research to suggest work is good for one's mental health, there is also plenty of research to suggest children benefit from a close, bonding relationship with their mother in the crucial first three years of their life.
ScottishMummy - you clearly have such a huge vested interest in the former since you cannot stop yourself from entering these threads and banging the same old drum.
I have no vested interest either way - my youngest will be 2 in a couple of months, I'm a SAHM about to return to work full time as I'm not cut out for it and don't think I'm brilliant at it, although I adore my children and do my best.
Yes, working is good for one's mental health. We're all good at something and many of us are lucky enough to have vocations in certain areas. Some people are natural mothers. Surely someone who is a natural mother, or really enjoys mothering and looking after young chidren - and there are plenty of these people - will be better off doing that, rather than entering paid employment, simply because ScottishMummy from Mumsnet says she would be setting a better role model for her children if she left the house to earn a wage.
I'm sure we can at least agree her children would be better off. Yes, those inconsequential people who get in the way of earning a crust... 
Unfortunately I'm not cut out for full time stay-at-home motherhood, but I feel no need to beat some beleaguered drum that says no-one else should do it either. That I'd be positively doing my children a disservice by being a SAHM. The notion is laughable.
It always reads utterly like you're so desperate to convince yourself of your stance; no-one else.
I rarely post on nursery-related threads because there is no benefit to making people feel bad about 'choices' they sometimes have to make.
I did return to work after DS turned 1. We explored lots of nurseries and I didn't come away from a single one feeling like I could leave him in it. We went down a different childcare route entirely and will be again when I return to work in a coule of months time. Luckily for us we were/are able to do that; I fully realise many are not.
As for why people who work in them don't improve the system from within - well, honestly, I don't see how they could. They are often young, inexperienced, unable to change ingrained cultures, outnumbered, etc, etc. The list is endless.