If you use a shower that other people use, it's just rude to piss where they are gonna stand.
You may as well wipe your snot on to the back of their shirt while you're at it.
And if you do have a shower no one else uses... Your still a lazy son of a bitch if you can't use the toilet.
How long are you in that shower for? Do you not need to piss before you go in there?
"Uh oh, I've been in this shower for so long that all of a sudden I need to pee. I'm gonna be another 30 minutes in here, so I may as well just do it because I'm too fucking lazy to use the fucking toilet"