I disagree. A house rule mutually agreed upon actually has
more validity than a Perry-Sez. It even trumps the rulebook. You're selling your argument short a little bit here.
But in general, the collection of Perry-Sez are a useful tool for resolving rules disputes. If you have a dispute with your opponent on a rule not covered by a house rule, and your opponent points out a Perry-Sez that directly addresses the issue and supports his side of the argument, are you saying you would not even consider it?