They're a fairly inexpensive way to add space to the house, but do need to be planned carefully so that they are actually used. They're usually not on the central heating system, so electric heating which is expensive - I've got some friends who didn't factor this in and they say it costs them a fair bit keeping it warm for the workday in conditions like we've had last few days, as even with insulation and double glazing etc, it's still an unheard building with 4 external walls & a roof exposed to the elements.
Yeah they won't last long term but a well built one you should get 20 years out of, a properly built outbuilding would cost much more.
Agree that having a garden bar is generally a waste, I've seen ones where sole have actual pumps put in, who wants all that chew? Perfect draft etc is an option, but they cost quite a lot in electricity to run and the beer isn't that cheap either. But for extra work space, working out space, summer socialising space etc they can be good. Not everyone has a home office or big enough space for those things.