hardocp already found that having more than 3gb was useful if running sli on titan at high res 8 months ago. its STUPID as hell to release a fully enabled GK110 at this point with only 3gb of vram. again many people that get these cards will be wanting to run sli at really high resolutions.
I can see both arguments. I'm not convinced that 6GB is needed,because Guru3d tested 4k resolutions with ultra/MSAA and not even BF4 approached 3GB of VRAM usage. Then again, we argued about 2GB a year ago and many games are surpassing 2GB of VRAM at 4k resolution. So, it's a tough call - the future CAN change things, but at this point and time I cannot see more than 3/4GB being useful. I have not seen any real world test at any resolution showing anywhere near 6GB of VRAM use. Yet in the back of my head I keep thinking of the 2GB arguments we had a year ago. Heh. 2GB is worthless at 4k resolution, but of course not a lot of people use 4k.
For the more common 1080p-1600p resolutions, 6GB may be worthwhile at some point in the future, that's a very big if. Right now, though? It isn't. 3GB is plenty up to 1600p. Keep in mind that the Titan also doubles as a CUDA development card and the 6GB of VRAM could be use there, but I cant' see it being entirely useful for games. This could all well change a year or two for now, we'll see. In any case, it's more or less confirmed that the baseline 780ti will be 3GB, so it's really a moot point to argue.
I think a lot of this depends highly on the resolution in use. Certainly someone at 1080p or even 1600p won't need to bother with 6GB. OTOH, someone running 1600p*3 may find 6GB useful. Therefore it depends highly on the user in question. And those users that are running 1600p*3 will generally not be stupid enough to buy VRAM deficient cards - they will have options for higher VRAM configurations.