Like many of Apple's rules, this appears to be aimed at making illegal sharing just *a bit* more difficult. You can temporarily authorize your friend's computer to play your music, but you can't use Match to get the music there. Of course you can still use a thumb drive, hard disk, or generic cloud service. As always, it's trivial to break Apple encryption anyway, if that's your goal.
So this doesn't really limit illegal sharing, but can be a really inconvenient for legitimate, paying customers. In your case, they would only make money if you *bought a second computer* for your wife's content. As it is, they've lost even the $25/year Match fee.
Any concerns could be addressed by tightening up the rules on authorization (say, limit the number of times a single computer can be de-authorized and re-authorized), but maybe there are legal problems doing that after the fact (OK, I admit, I've never read the 60 page agreement when I buy stuff).
In a few years, we'll all have enough generic cloud space that we can just put all our music there and make this a non-issue. In the mean time, it's really annoying.