IMC, the Tarrasque has a big role to play as an Epic-Level challenge. Y'see, he's got a mad king begging for his resurrection and a world to rampage.
Basically, I see the Tarrasque as The Godzilla Factor. He can level any pansy commoners that try to hit him with tanks and shotguns and stuff. He can stomp buildings and kill even big monsters, no prob.
But who do you call when Godzilla gets out of hand? Well, that's when you get Mothra (e.g.: epic-level PC's) to kick some almighty booty.
And, yeah, I'm not against givin' ol' Terry some levels and stuff, either. Templates are iffy....vampire Terrasque? Eep....