Firstly, in order to properly handcuff someone, you very likely will need two hands (one to hold the cuffs, one to hang onto the arm being cuffed). This means putting your pistol away. I'm in a law enforcement job, and I would be a bit uncomfortable by myself, with a dangerous subject, and nothing more than handcuffs dangling from my fingers. That's why we like backup.
Second, there are proper ways of cuffing, and improper ways. Do it right, and things can still get hairy on you if the subject decides it's "go time." Do it wrong, and you give the subject a real good opportunity to lay hands on you.
Finally, once the subject is cuffed, he's your responsibility. If the cuffs are too tight and restrict blood flow to his hands, you're liable. If he tweaks his wrist, you're liable. If he gets up and runs off a cliff, well, he was handcuffed, so why didn't you stop him? It might not make the best sense, but this is how my instructors trained us, and how lawyers are likely to approach such scenarios. In this litigious society, even if you win the lawsuit, you may well lose in lawyer's fees.
As an afterthought, consider also the public perception. While this is lesser than the three items above, a non-LEO carrying cuffs may appear to be a "wanna-be" to the public, and a loose cannon or crazy as a result. (Mall ninjas, anyone?
) For what it's worth, I don't carry my cuffs when I'm off-duty, either. Just my gun and creds.
Just my $0.02.
Thane.