The primary benefit for more VRAM is more anti aliasing at higher resolutions. Or for professional applications. Professional apps can absolutely use more VRAM. Gaming though? AA absolutely takes more VRAM in current games than basically anything else, given that game assets have been at a standstill (relatively speaking) in the past year. Such as surround and 4k resolutions - MSAA/SSAA takes far more VRAM than basically anything else, including game assets or textures. Textures, in fact, are fairly trivial in most games. Even 2GB is generally fine up to 5760*1200, tested by numerous websites.
The thing with VRAM though is that SSAA kills VRAM use. But then again, you run into situations where SSAA is so demanding that you run out of GPU power before the VRAM wall becomes an issue. So if you want to use 8X SGSSAA, downsampling, resolution scaling (same as downsampling), then you want more VRAM. FXAA uses zero VRAM, 2X MSAA uses very little VRAM, 8X MSAA uses a lot of VRAM, SGSSAA uses a ridiculous amount of VRAM, and OGSSAA/resolution scaling / downsampling (all the same thing) uses a god-awful amount of VRAM.
If you want more anti aliasing, get more VRAM, if you use 4k or surround. Personally, if I were at 4k resolution i'd be fine with FXAA or 2X MSAA. But that's me. I really don't see why people go nuts with AA because I can't really spot differences past 2X MSAA, and while SSAA is better and noticeably so - the performance hit isn't worth it most of the time. But whatever people want. Their money, they can buy whatever VRAM they want. I almost feel like the VRAM wars are a near psychological benefit much like the megapixel wars with smartphones. More doesn't mean better, but it really does depend. With 3GB you're more than set (up to 1600p or 5760*1200), but if you use 8X SGSSAA maybe you can't do that in triple 4k resolution. Or 4k resolution. More VRAM would be beneficial for 4k if you like using a lot of anti aliasing, and 4k has a higher pixel count than 5760*1200. Then again, SSAA gives you a 70% performance hit compared to 2X MSSAA. Worth it? Nah. Screw that. I'll take 2X MSAA for 70% better performance. Or FXAA for zero performance hit and zero VRAM use. But that's all in the eye of the beholder.
I don't even care about 6GB cards still. If I were on the market buying new i'd still get an on sale 3GB 780 custom card. I monitor my VRAM use constantly with afterburner, and I play a lot of games at 1600p, and it's just not an issue. And I don't use tons of anti aliasing past 2 or maybe 4X MSAA. I use SSAA in older games, but crysis 3? No thanks.
That said, I think this is cool. 50$ for 3 more GB of VRAM isn't all too bad and it's more choice for those who want it. If they want more VRAM and want to spend their money that way, hey great. More choice for consumers, what's not to like.