Had the GTX780 came with 6GB as default instead of 3GB, it would give far more incentive for people (myself included) to consider buying it.
It's difficult to consider buying GTX780 from financial perspective when a 290 is at £260-£300, and the GTX780 3GB is on £360+ over average and the recently launched 6GB version at £420+.
If it was the GTX780 6GB at £360 ish (still nearly £100 dearer than the 290) rather than the 3GB version, it would at let people feel like they are actually getting something extra for the premium they are paying.
Think there's at least one or two occassion that I've seen members claiming they are running out of vram and performance suffering at 2560 res. Disagree as you may, but the corner that Nvidia cut on vram DOES make the GTX780 age worse than needed to be, comparing to the Titan with 6GB of vram. One of the reason why the Titan aged so well is not because of its compute capability, but the extra vram it has over the newer GTX780 3GB...