I am sort of a landlord. Well i rent my mum's house for her, so she gets the cash but I get the aggro. Works well....for her! lol
Anyway, I agree that landlords don't make vast amounts of money. If my mum's place had a mortgage, we'd be running at a loss most months.
People see the rent and think it's all profit. It rarely is. There's all the regulations, certificates, insurance to pay for. After that there's the repairs which seem to crop up quite regularly. There's always a leak or something broken or whatever.
The reason why landlord's don't like accepting HB claimants is simple. Firstly there are quite a lot who would happily trash the place, not pay rent then piss off into the sunset. Of course a professional couple can also do this but its considered less likely. I've not done the figures myself but the key thing is that the perception is that a professional couple will be a better bet, especially if they can afford to pay more.
Secondly council's can claw back housing benefit paid directly to the landlord. If the tenant pays then they cannot but then you go back to the risk of them not paying again.
A good friend of mine is a single parent and her landlord was initially reluctant to take her on since she was on HB and had been stung last time by the council before his old tenant trashed the place. She's paid up without fail and when the HB messed up, even took an overdraft to ensure rent was paid on time. These types are like the holy grail of tenants. As a result her landlord has frozen rent rather than risk raising it and her moving out.
My point is that most landlords don't make anywhere near the money most think and you really have to see how many idiots there are out there who are renting to see why landlords are so selective. When we do find a good tenant, they are worth their weight in gold and most will do their utmost to keep them happy.