There's a reason Time Lords only get a certain number of Regenerations. (Yes, yes, it's been forever, where have I been? Does it matter? Moving on...) I've heard plenty of theories: the cynical, that the Council of Time Lords doles them out; the logical, that it's a biological restriction; and the intriguing, that it's actually due to some law of conservation of Vortex energy or something, and that with the rest of them locked away, the Doctor has access to all those unused Regenerations.
But I think it's simpler than that, and more interesting, especially for someone like the Doctor. Hell, he's even said it himself- he's tired, old and tired. He's getting on towards 1,000 years old, and it's been a life of near-constant chaos, danger, and intrigue. It's very much like the Heinlein-Martian idea that a city can fill with memories and need to be left behind. For other Time Lords, by the time they get to that 13th Regeneration, they've probably lived closer to four or five millennia, maybe even more; they get into a lot less trouble, overall, than he does. They've still got an enormous amount of experiences and memories, and they all get a bit tired. I think that, after everything he's seen and done, even if he had the option to take on everyone else's spare Regenerations, he wouldn't, not if he had any other choice.
Which is part of why I think they need to bring back Romana. For one thing, I think it would do the Doctor a world (or two) of good to have some family again, a friend who understands about Gallifrey and the loss of a people, an ally who can always keep up with him. Also, he's gotten a bit reckless, and I think she would rein him in expertly, keep him alive just a bit longer. But the other side of that coin is that he would have an heir, someone who can continue his work when he uses up his lives and can finally stop running. She has plenty of experience keeping up with his quick-turns, both mental and physical, and she believes in the goodness of sentient beings and in giving everyone one last chance to mend their ways. There's no one in the Universe he's more likely to trust to continue his legacy, and no one more qualified.
Lastly, one thought on a slightly different topic. Interspersed with the Eleventh Doctor stuff I've been watching, I've also gone back and watched some First Doctor, and I'm intrigued by the similarities. Anyone else with me in thinking that, before we ever met him, the Doctor was as much of a poker and a pryer and an annoyance as Amy ever found him? Granted, he was likely a bit less wacky, what with being surrounded with other Time Lords, but some of his current oddness may be senility anyway. Or at least the weight of age... *This thought inspired by Teenage Rebel by Chameleon Circuit, from the album Still Got Legs.