This is something that I have experienced recently myself.
I work full time at the moment, doing 33+ hours a week at £6.73/hour doing a job that warrant's much more because of its physical strength requirement (lots of heavy lifting).
My girlfriend recently moved in with me, she is unemployed and unable to work due to her health, she gets DLA and prior to moving in with me was getting £700 a month in benefits.
Now I take home around £200 a week, I also claimed working tax credits which topped that up to around £240 a week and housing benefit of £17 a week, putting me at just shy of £260 a week. My rent and bills come to approx £670/month more than half of my income, this is the cheapest I can get in the area living on my own, I could chose to go into a bedsit or rent a room, but at 34 the last thing I want to be clearing up after messy house mates specially after working a 10 hour shift and getting home at 11pm.
Anyway with my girlfriend now moving in she has lost her benefits due to my "income" which can barely sustain myself, she still gets her DLA but that is it. My working tax credits have gone up slightly as well as my housing benefit but it is still a fraction of what we were getting before living separately, but i wouldn't be coming home knackered every night either.
And the really amusing thing about this all, if I stopped working completely and just scrounged off the government my income would actually be only £30 a week worse off than I do working currently.