I have participated in a couple of giveaways and felt that they were not particularly helpful. I did one through the HandmadeMN blog, which is a regular thing that team does, and I promoted it on various sites and my Facebook page etc., and there were a decent number of responses, but in the end, it only attracted people who like giveaways! At least, that's how it felt to me. So, although the giveaway itself got plenty of "business," my shop did not benefit.
I think you're right that if you don't do a good job of promoting your giveaway, you don't get much from it. But that's true of our shops in general, so why spend time and energy promoting something you're giving away for nothing, instead of spending the same time and energy promoting your shop?
I just think that when you give stuff away, unless you're donating it to charity, you are saying something about the value of what you make.
Anyway, that's my two cents' worth!