Scenario: Legendaries are gym defenders

Lugia has 100 points more def (but less HP) than the pink wall AND a middlingly decent atk stat. Could be a problem.

1 Mythical and 1 Legendary should be allowed in gyms. First come, first serve

it'd be interesting to see, you also forgot about Latios, Rayquaza and latias also great for stopping machamp

I'd like to see it happen, give a lot more use to some of the less used legendaries, suicune and the regis are mainly defensive and on offense they arent much use, but on defense they could be good (well, suicune maybe) then theres gen IV with cresselia and the dragons, so it would really change the meta if they did, would add a lot more variety to what we see

Unless we limit to one legendary per gym, the only defenders we would see are legendaries and Normal-type tanks.

Lesser legendaries should be allowed in gym (those with 580 total base stats in main game like the trios) afterall they are allowed in battle tower in the main game as well. The greater legendaries should remain banned. That way, the ym system would be largely intact, people wont find it frustrating seeing a legion of lugia hooh mewtwo and such in every gyms, defensive currently useless legendaries find their purpose. Every one happy

Legendaries are pokemon with free will. They can't be tamed. They wouldn't stay in a gym just because you wanted to place them there. That's why Niantic doesn't let us. To avoid disappointment

If you could put as many legendaries and/or mythicals as you wanted, the optimal gym lineup would start with Blissey-Lugia-Chansey, followed by Snorlax/Latias/Ho-Oh/Gardevoir/Mewtwo/Raikou/Kyogre/Mew/Celebi in some order. (EDIT: Maybe not Gardy if Machamp sees less use)

In areas with lots of powered up legendaries, there probably would be less Machamps in gym teams. Raikou and other electric types would rank higher since they can cover Lugia, Ho-Oh and Kyogre. Rock Ttar would obviously be a great pick as well since Lugia would be an autoinclude in gyms and you'd need something to counter it, and it absolutely destroys Ho-Oh.

Only one legendary per gym, end of story

With the Motivation system in place, Legendary Pokémon are not nearly as game breaking as they could theoretically be. And even when you only consider Full Motivation Pokémon for this discussion, very few Legendary's are going to be causing major issues for any player who has put any effort into making good anti-Legendary teams.

Bring on the 5 Legendary Defenders who will still pale in comparison to Blissey!