It is.. well being generous so.. guild level is something everyone can see and verify. You can't claim a guild level you don't have. Anyone can claim to be the first level 20 half-orc, but how would anyone prove that? but you can claim to have for instance a level 70 guild, and anyone from the person who joined on the first day to the person who's played for years, can see whether or not your guild is level 70.
So it's an independent, verifiable metric of measurement that is independent of opinion and conjecture.