Sigh the 780ti cannot use more than 4GB Vram let alone 8GB Vram. Don't fall into this Vram hype because it is absolutely BS that has not even been very well quantified yet.
First this is complete mis-information, why? Because it would use 3, 6 or as some rumors suggest 12GB of vram, not 4 or 8.
Second, yes you very much can use that much vram. Yes, chances are that at 1080p right now 3GB is fine, but for how much longer into the future, let alone 1440p, 1600p, and beyond. This is very apparent if you happen to mod a game with high res textures like you can with games like Skyrim. Hell it is even more apparent if you start talking about 4k displays which while pretty rare now, but won't be forever.
Point is while 3GB may be enough for you, it will not be enough for everyone.