Reshiram and Zekrom actually have a higher catch rate than most (if not all) other legendaries, because you're required to catch them to advance the storyline. They can't even be Shiny, so there's no risk of accidentally knocking out a Shiny Reshiram or Zekrom. Sorry to say, I think you really wasted the Master Ball there.I used mine on Zekrom. Was it worth it?
I don't agree with using them on the roamers, actually. It's true that any other Pokémon can be caught with enough Poké Balls, but you gotta plan for such eventualities as Shinies that know Explosion/Selfdestruct/Perish Song/any recoil move (unless it has Rock Head). Or Shiny Metang/Metagross/legendaries. For roamers, my personal favorite strategy is to put them to sleep and wear them down to 1 HP (obviously would have to be done over the course of several battles), then just chucking a Quick Ball at the beginning of each subsequent encounter. This way, you can get the benefits of a Quick Ball on an already weakened Pokémon, which is normally impossible. It's how I caught Thundurus last month.I agree with using it on the roamers, any other pokemone can be caught with enough pokeballs, the roamers could potentially take you the longest to catch by having to track it down each and every time, unless you shadowtag or mean look it.