I have a tasting set, and they're really quite nice. It's a bit thicker than a gaiwan, which makes it a bit better for darker teas, it would certainly be a whole lot easier to carry around and use at any kind of event, the cup is particularly nice, and the spoon both gives you a much easier way to really smell the aroma as well as very quickly cools very hot tea so you don't burn your tongue (even moreso than normal spoons). The spoon also makes it easier to slurp the tea in the way you need to for a proper tasting.
You might try taking a spoon, dip it in some tea, hold it about 4-6 inches above the teacup while you take a whiff from the spoon - I've found that you can get a lot more from the aroma than any other way. I'm sure this is in part due to the fact that you're also getting some of the aroma that you otherwise wouldn't be able to get until you reach the bottom of the cup.
I forget I have it sometimes, but when I do I will use it especially for anything I'm having a hard time getting right. Although you might be able to get the same results from a gaiwan, you can usually get those results more easily with the tasting sets.
And lastly no, it's not a gimmick. If you check out pictures from professional testers and some of the events you will often see a row of these sets with a different tea brewed in each. If I remember right, the History Channel documentary showed the company taster at Lipton using them to check the various blends.