Thanks very much for the input. It seems to confirm that there is something wrong with the battery, but that the original Subaru battery was crap to begin with. I've contacted Subaru again to see if I can get satisfaction from them. If that doesn't work (and I do think they should do something), I'll start looking into the other solutions proposed (battery tender or new battery).
It's a shame, though. I and friends have owned a lot of Subarus, and this is the first real problem I've had.
If you don't have an extended warranty, I'm not sure what "satisfaction" you expect to get, and batteries are typically an excluded wear part in every car ever made except for battery powered cars.
Leaving your accessories/lights on and killing the battery is not a manufacturing defect.
That will happen to any battery in any car.
Nothing said on the replies actually confirms anything - anecdotal testimony is not a confirmation of what your problem is... but frequent discharges to a "dead" state by leaving your lights on will cause damage.
I noticed you never mentioned charging your battery after it went dead.
Batteries that go dead should ALWAYS be recharged to prevent damage.
If your battery stress test results are okay, replacing it is no guarantee of anything except spending money on another battery.
If your battery is marginal, you might be okay with a replacement, but this is not for certain to address an unknown issue.
A battery with a higher amperage will take longer to discharge, but it will also go dead if there is a significant drain on it.
A parasitic drain test will easily identify that issue. It is a very simple test, and you should do that before you replace a battery, or risk killing it.