They are useful for people who want a gaming PC but are always moving, such as students who live part time in a student residence and part time at their parents, or cases like that.I personally could never see a reason for a gaming laptop. Sure, they might run games well, but in what situation was I going to find myself out of my house and not occupied enough for me to play games, even more so the ones that actually require a graphics card to play. This might be different for you, but it's worth asking yourself if you will actually use the added functionality it gives, or you have no intention of getting a desktop.
But yeah, the power/price ratio is a lot lower, so everyone who can avoid them (which isn't necessarily everyone) should.