I really can't see many people having a real problem with this. I know that, barring some wacky, out-there circumstance, whether a parent had a partner to hold his/her place in line or not, I'd let them back in, especially if we'd been waiting for a long period of time. That's just common courtesy.
I think, like with a lot of things in Disney, that the expectation that someone should or will do something is not the best attitude to have. I believe that, yeah, while it would not be a big thing to me, everyone is not like me. So, if I had to leave, I'd ask a CM and hope he'd be accomodating, but I wouldn't expect that he or she should. Line jumping is line jumping, after all.
However, under no circumstances is it okay to whip out a toilet just to escape leaving the line. Last year, I took my DN7 and DS9 to Six Flags. I know that my nephew has a small bladder, so DBF and I were very diligent about constant trips to the restrooms, whether they were requested or not. Right after one such bathroom break, as we'd been standing in line for a ride for about 30 minutes, DN has to go. "Really bad, Auntie!" There were no employees in sight so I asked the people behind me if it would be a problem and everyone was really nice about it, when they didn't have to be and there were no problems. There's no way that DN would have been allowed to relieve himself publicly, not at at 7, 5, or 2. Just not cool.