When I was a single parent on IS I would have only been £10 a week better off. Not much of an incentive to go back to work full time and barely see my children.
Sadly that mirrors what a lot of people find - there isn't enough of a reward from the wages to offset the "extra" benefits you can't really put a price on easily... free glasses, prescriptions, dentist, school meals, hell, when I was on partial benefits for a year due to illness, later, i found out i would have been entitled to quite a few other things that I wasn't aware of. Apparently if you show proof of being on certain benefits to some places they have concession rates you can pay. I never knew this (maybe it didn't apply at the time i was on them, though).
I've forgot free tuition for courses in that too.
and then you have to factor in things like commuting costs, work clothes, packed lunches or extra costs like needing to run a car often too (i'm very lucky in that we don't need a car for my work, as it's within walking distance - just). and the stress of a job or not getting to raise your kids yourself on top.
I wish i had the numbers from when DH was working part time (when I was ill) to support us, but i was shocked. it was something like:
Council tax benefit £15 (we had a few quid to pay on top)
Tax Credits £180 ish
DH Wages at least 2, but often, 3 days a week - minimum wage at the time so £135 ish for the 3 days he did
Child benefit £50 (obviously they've been making changes to this recently)
Housing Benefit £145 (I think we were paying £10 or £20 on top)
WTC £15 (ish)
I mean, if you tally it all up, we ended up with far far less disposable income (in cash terms) by being in that very stressful and upsetting position, BUT in terms of household income, that was like being on about a £26k a year salary when you calculated it all out - neither of us have ever earnt that, and it was only DH working then. I'm working and well now, but that fact has been in the back of my mind since. (esp. on days when DH and I get in late and barely see the kids before bedtime).
Employment clearly brings in more than just material rewards - self esteem, showing a good work ethic in the household, chance for promotion, pension contributions - but if that difference between not working and working is so miniscule as to be pointless (whilst not being there to see your children grow up), I honestly don't blame people for choosing the more logical option.