I used to like that they can't be put in gyms because I didn't have any legendaries. But now that I have two (so far; Suicune and Groudon), I would like to show them off in gyms... but then again... so would others, and then they wouldn't be special or unique - in fact, there would be nothing but legendaries in gyms, I'm afraid.
There must be a reason for the madness (right, Niantic?)
Legendaries should ABSOLUTELY be able to defend gyms. Even if you make a limit of how many legendaries can be placed in a gym or something, it's ridiculous to me that legendaries can't.
With the overabundance of raids, so many players have legendaries now. And with such powerful pokemon, it's never been easier to take down gyms. Any player can take down any gym now. Adding legendaries might add some difficulty to gyms again.
Furthermore, there are many legendary pokemon that are BUILT for defense, with tremendous defensive stats. Lugia and Suicune, for instance, would see a new lease on life. Both have good defensive movesets and could see some real usefulness in the defensive sphere.
The only thing I can think of is because not every player is able to do legendary raids (e.g., rural). That way they don't know what they're missing. I know, because I used to hate the fact that I couldn't get them and was very relieved not to be confronted with them in gyms (not because I didn't think I could take them on, I have great attackers). I was just jealous as hell!
But that's just me, no telling what Niantic's reasoning may be ;)
I never understood this rule in the first place. The only one of each kind rule is somehow understandable (diversity) but Legendaries aren‘t that much stronger and 6 vs 6 - even if they are at max which they almost never are cuz motivation - is no challenge for anybody.
Would be cool to change that and go back to the 10 defender system without training which favors spoofers too much
I think it's:
1. To prevent newcomers from being envious or gyms deterring newcomers under first impression in spite most legendaries don't make good defenders;
2. To prevent Cheating Legendary candy by feeding berries.
So it might not be able to happen unless when the game is infested with op pokemon which will someday happen, in which i'm ok with 1 legendary.