Only casuals have problems with roaming legendaries. In fact, they're probably some of the most fun legendaries to catch. Every time you encounter them, it's the first turn of the battle, so Quick Balls are always at 4x effectiveness against them, 5x in some generations. Not to mention that they don't take status damage due to fleeing every turn of the encounter, so you can safely inflict them with burn/toxic/poison for a 1.5x boost in capture rate on top of the 4-5x boost from the Quick Ball. Oh, and they never attack unless trapped, so you can safely status them, too.

The fact that people have trouble catching Roaming Legendaries is a testament to how shit they are at a children's game.