If you look at it that way, your arguement makes sense.
If I was buying a GPU I would be looking for the larger memory/wider bus for the simple reason that I would want to go SLI/Xfire at high resolution.
Of course, it makes no sense at all to buy cards with lesser RAM and memory bandwidth when you're talking similar money.
That aside, think about it, when the cards are getting a bit old, the ones with 3GB RAM and a 384 bit bus will be able to run games at greater memory intensive settings, higher res textures, higher res and so on.
You'd have to lower the settings more, a fair amount more on a card with 2GB of RAM and 256bit bus than you will on a 3GB card with 384bit bus.