Cecil’s answer to this question was very comprehensive. My feeling is this: you need one good glass for reds, and one for whites. The shapes and sizes will be slightly different because of the variation in body, aroma, and other characteristics of fine wines.
I’ve NEVER understood why wine shops that do tastings give the product to you in little plastic or paper cups, because wine just plain doesn’t taste right in them. You do need the proper shape to aerate the wine when you swirl and to direct the liquid to the right places on your tongue. A comparative tasting with Georg and Max Riedel convinced me of that.
However, there are some specialized wine glasses that are worth having on the shelf. I like the Helicium glass that I discovered at VinExpo in Bordeaux a few years ago. It has raised vertical ridges around the bottom of the bowl that help to aerate the wine when you swirl, and bring out the flavor…sort of like instant decanting.
Author of “Secrets of the Wine Whisperer”