It's a small price to pay when the alternative is divorce, which is usually due to the financially burden of having to look after a family on a shit entry level salary. I know people it's worked for and you know people it hasn't worked for, it's all anecdotal but I think it's a better than the shit we have at the moement.
I understand that Islamically he's responsible for her, but that doesn't absolve me of my responsiblity as a parent. Times are hard financially and they'll only get harder by the time I have a daughter who's of age. I don't want my daughters to engage in zina, so I see this as the perfect compromise. Besides, it wouldn't be indefinitely, it's just until they can get on their feet. You mentioned single mother, but that's exactly what she'll become if I don't step in and ease the financial strain.
Most people have to have roommates, that's how fucked the housing situations is in most mid-size cities. Why put that burden on some poor uni student who's probably not going to make enough money to support a family for the next 5-10years? Parents need to step in and take some responsiblity if they want their children's marriage to work wallahi. Because letting them struggle is only going to cause their marriage to implode