I think it depends of every character, some may use legendaries because they think the situation is necessary, some may don't care at all and crush a little Weedle with their legendary, others may just adjust for a fair fight. We already have people that have shown that "regular" pokemons can defeat legendaries, so I think it depends of the character, if it has a good fundament of having said legendary. For me personally I'm the type who like to adjust to the situation, if my opponent is clearly weaker I won't shove them a legendary right at the start