Where I would concede that it seems ludicrous is on the Mac which has the capability of multiple users. I don't know maybe we're weird?
That's pretty much the case everyone's talking about here.
First of all, this policy does absolutely nothing to stop illegal content sharing. You can illegally copy files in dozens of other ways, and just about *no one* is stupid enough to share conent by giving *someone else* access to their iTunes account, so that's a complete red herring. It's just an annoyance to loyal customers who are trying to follow the rules. If it made sense "from a business standpoint", then Amazon would do it, and they don't.
Second, you and your wife apparently met *before* you had iTunes libraries. When my wife and I met, we both had large libraries (which of course can't be merged). My kids also have their own accounts, so they can learn to manage their own funds, rather than just charging things to Dad's credit card. That's four people in the house who just want to use shared computers (like the one connected to the entertainment center) to access stuff they've legally bought and paid for. Apple doesn't *prevent* it; they just make it a pain in the ***, which is a really lousy way to treat people who have dumped much money in their pockets as we have.
For example, my son came home from college and redownloaded a movie he had bought on his laptop. This action locked *me* out of using my own Match library for 90 days (or would have, if Apple hadn't graciously reset the counter).
Of course, he could have moved it over on a thumb drive, or by remotely mounting the disk, so again this has nothing whatsoever to do with illegally sharing content and everything to do with being annoying.