Yup, I purposely picked the level 9 cell instead of the level 10, so it would generally include a couple more towns, or, in the case of metropoles, reach all the way to the other side of the city. Which means, if people are dead set on putting a specific Legenadary as a defender, they’d have to remove it elsewhere first. Or, alternatively, cooperate with others to remove it elsewhere for them.
Only a few Legendaries would make really good defenders that could somewhat compete with the likes of Blissey and Snorlax. So after a while, gym defense would naturally gravitate to those, and in many areas, you’d quickly end up with every gym filled up with Blissey - Latias - Snorlax - Lugia - Vaporeon - Chansey.
Having a limitation that makes placing a legendary less common, will make people happily take the opportunity to place a less ideal Legendary, just to show what they have, keeping gyms somewhat varied. And keeping the defenders varied in turn makes it possible to feed those legendaries that you’d like to get some candy for.