With 1GB becoming standard on modern GPUs, it is a very, very small piece of the pie in the grand scheme of things. Yes, if the extra buffer causes your card to use more than its dedicated memory pool, it may not be worth it, but this will be a very rare example on modern cards. 512MB cards should be fine to enable tripple buffering as well, outside of Crysis and extreme resolutions perhaps.
This is the reason why its not widespread on the console side, any spare chunk of memory is precious when you're in a RAM starved environment like that. However, the hit is in the single digits at 720p, and personally I feel that is more than worth the tradeoff. RE5 is one example of a PS3 game confirmed to be using triple buffering and it has some very high resolution textures. Its also believed this is what Naughty Dog is doing with Uncharted 2, and that game has the best textures on consoles, so if this is indeed the case it proves that enabling triple buffering on consoles doesn't have to mean compromising your texture budget to any noticeable degree.
I hate, hate, hate tearing, so I would like to hope more console developers (at least on the PS3 side where implementation is straight forward) give it a go.