I disagree a bit. If they have made up their mind, or did not do that well, yes for sure, your not going to change it.
If they have two candidates they can't decide between the two - then it might help.
Can also depend on the job. If its a sales position - even if they wanted to hire you they would not if you didn't follow up. Its a bit of a test.
If you interviewed well but are just a bad fit for that job, they might hold on to your resume or re-interview in the future.
Very job dependant, but it doesn't hurt.
Yes, I was a hiring manager for years - so this is from that side of the desk.