I'm not trying to brag here by what I'm about to explain. And I'm sure there are many here who understand why sometimes you just need to piss in the sink.
If you've ever lived in a flop house, you know the room: There is a bed, a steam heater, and a sink... There is no toilet.
In addition to it being convenient, that sink is a whole lot more safe in this enviroment. Flop houses are very dangerous places. I knew guys who had three locks on their door. Partly because they didn't want to get ripped off while they were out during the day, but mostly because they were scared at night -- as well they should have been.
So you go to bed drunk, knowing that it's the only way to sleep during the night in one of these places. But you still wake up, to crazy men who shout at empty walls and women screaming because they are about to get rapped or beat. You listen to it for a bit the first few nights, then you do your best to not care.
Your four walls with the sink are still safe, even with only one lock on the door. But if you venture out into the hallway half awake and still drunk and try to make it to the shared bathroom, you're an easy mark and liable to get robbed, beat, ass-rapped or otherwise.
So, to come full circle, that's why it's better to just piss in the sink...