It is "only one more person" because you assume only one person is doing it. If the rule is that everyone can let one member of their party do something else (like get fastpasses, drinks, ride something else) while their party waits in line for them, then we would all do it. If the rule is that you cannot do it, then it is not fair for someone to do it and claim "it is just one person".....since it is "only one person" because the rest of us respect the rules and each others interest in keeping the wait in line manageable (by not allowing cutting).
In sum, if saving a place for a party member is OK for one, it is OK for all. Since it won't work for all, then it is not fair for one party to decide it is OK for them to break the rule since everyone else is following the rules.

I don't think I am making my point well.
The answer to your question is yes. Period.