My friends used to have a small business hiring out their 3.5t box.
It was popular, covered the running costs I think, and they managed to sell the box and business as a going concern.
I would say the major pitfalls are:
The main times when people want to hire it (weekends) are likely to be when you want to use it yourself.
People just don't respect other people's property unfortunately, my friends found their lorry sustained quite a bit of damage, people wouldn't even bother to muck it out on returning it and very rarely came clean about damage they'd caused.
Insurance is expensive, as you have to have proper hire and reward insurance as well as insurance for any driver.
Going from their experience I would say that unless you are starting a proper hire business (as opposed to simply hiring out your own box when you're not using it) I think it is more trouble than it's worth! Any money you make is probably spent on repairs and increased maintenance.