Ok, maybe I'm not done yet. Cool, then tell me what your "reasonable balance" is. How many seats can I save and for how long can I save them?
It doesn't actually matter. It doesn't need to be (and indeed, cannot be) specifically defined. If we read the letter of the rule in the strictest fashion, you are right. Only an ass saves a seat. Absolutely nothing else technically saves a seat. But in practice, the non-specific, unscientific, "don't be an asshole" commandment, if followed, will basically make things work smoothly. Despite the fact that different people may interpret it in somewhat different ways.
I mean, really... no one who wants to sit down is running around aggressively targeting the seat of someone's spouse who went to the bathroom. If you sit in a seat that doesn't have an ass in it, and no one objects, you're fine... whether you had to move a bag or not in order to sit there. If people later return and want to reclaim those seats, you're well within your rights to refuse. But in practice, a fair percentage of people will just move, because they really don't care that much (or were planning on leaving or moving to the floor or going to the bathroom or getting a beer soon anyway).
If you try to sit in a seat right next to someone, and they say they're saving that seat for their spouse, you are once again correct... technically, you have every right to tell that person to fuck off, and remain in that seat. However, in practice, most people (I'm guessing even you) don't want to spend the next several minutes or hours right next to someone who's seething with anger or resentment... especially when there are plenty of other seats that are either empty or have completely unattended bags or towels on them, that you can easily sit in.
This is a medium-sized venue, not a huge arena, but it's still big enough that in aggregate if everyone just does a quick 2-second self-check every now and then to make sure they haven't accidentally crossed the line into "being an asshole" territory, everything will settle into a nice equilibrium.
It's also a testament to how friendly the overall atmosphere of the fest is that despite the very strong disagreements that people obviously have over this issue, no major fights or conflicts have occurred over 14 years (unless there was something I don't know about at some point).
All I know is, I'm glad I like to be on the floor, so I don't have to get involved in the Seat Wars!