This has happened to me a couple of times now: I'll go out for a meal with some friends, and we all order whatever we fancy. I'll ask for something a little more modest, because I'm either not as hungry or because I don't particularly feel like spending too much money.
Then, the bill arrives and someone says "right, shall we just split between the four of us then?".
On both occasions, I just didn't want to bother arguing that I shouldn't be paying the same as everyone else because it's unfair. So I cough up the money and try to forget about it (though I'm writing this blog so I obviously can't).
Walking home today from campus, I was thinking about both sides of the argument. I basically saw one argument for each side of the debate. Those in favour of sharing the bill evenly, I presume, believe that we're all friends anyway so a little less or a little extra doesn't matter - it's a gesture of good will. Those who prefer not to split the bill evenly, such as myself, prefer to feel like we're being fair. I wouldn't want to feel like I was over-paying, and I wouldn't want someone else to pay extra just because I ordered a more expensive dish than them.
As I see it, neither one of these arguments actually refutes the other one. Rather, they simply both draw out different aspects out of the same social convention. So it's not as though two people having this debate could undermine the other persons argument with their own point. It's simply two things that are true. So it's a case of which truth you choose to focus on.
After having this thought, I wondered if that is the nature of all sorts of discussions and disagreements people have. They aren't actually disproving one another during a debate, they just focus on different aspects of the same thing.