I'd never take on a joint tenancy with somebody I wasn't married to - I've had relationships end where it went something like this;
'Well, you can fuck off, as I'm staying here whether you like it or not and you can sleep on the kitchen floor by the bin like the council trash whore you are'
OK, I'll call the police then.
'You fucking bitch. I'll make you sorry, I'm going to put you out on the street - you'll be hearing from my 'friend's' Dad - he's a barrister and he'll do anything I want'
Yeah, good luck with that. (picked up phone, got to 99 as he skulked out)
'blah..blah...blah...You can't make me leave, I live here, I have common law rights' (I have no way of conveying just how much he stunk of lager, could hardly stand up and the level of contempt for him that I felt at that moment)
No, you don't live here. You never did, you just refused to go home.
(big dramatic, faked suicide 'attempt' a few weeks later when he'd decided he was moving in officially and turned up with all his shit in his car, thinking I'd be so distraught, I'd immediately make him a joint tenant to keep him alive, as he put it)
If you're going to try pulling that shit, you can fucking well go and do it where I won't have to clean any of the mess up (sound of 14 stone being physically hurled out of the front door - slam).
Take on a joint credit credit agreement.
Take out a loan for something the other person wanted or to clear their personal debts.
Name as a beneficiary on Insurance policies or pension/death in service benefits.