I'm sure they must make vases in that shape, though, and that they are made as vases. Pottery is an ancient art form and they make all sorts of shapes for all sorts of reasons. There's no reason to say that shape X must only be used for decanting wine!
Jon - not to generalize TOO much but in general merlots that are sold "bulk" in 1.5 liter bottles aren't meant for aging, and therefore won't really be helped by decanting. If all she did was uncork it, nothing happened to it. I'm not sure decanting it would have helped - it probably would have hurt. And most decanters won't handle 1.5 liters, decanters are meant to hold one bottle which is 750ml.
So if she tends to drink casual wines, I wouldn't necessarily get her a decanter. It would hurt the wine, not help it, and they're a pain to clean
So it just adds more hassle.
Get her some nice wine glasses instead