They don't flee.

I wouldn't know but nice :D

some legendary pokemon run away from the middle of battle which makes them really hard to catch (cause you have to track them down across the entire world map again etc etc) so, even though they're not strong, trainers sometimes get pissed off and chuck hte master ball (which is like a 1 per game item that never fails to catch a pokemon) just to be done with it.

Ooooh that's a pain in the buns XD

Lol check dat attitude at the door mister cuddleywuffems!! :D

impossible! legendaries' attitudes are inversely preportional to their size.

HAHAHAHAHAHhHAHAHAHA
this.
Is.
Fact.
SO MUCH TRUTHS!

If they do throw one...then it makes you wonder if it's someone who cheated to get a LOT of Master Balls to throw at everything.

Admittedly...after like 13 times of going after Latios (or Latias can't remember which one it was) in like "Emerald"...I got tired of looking for it and not catching it. I threw the Master Ball at it. So...I'm kinda guilty of having done this. *lol*

Dialga doesn't flee the area as soon as it's encountered.

depending on the game, there is another way to get a master ball. (i know Soulsilver has a way. i think black/white and Black 2/White 2 have a way) but i don't think Diamond/Pearl/Platinum have a way.
But honestly, i spent 4 days trying to catch mewtwo in soulsilver without a masterball. So Dialga wouldn't be an issue. then again, i spent a week trying to catch Latios in Soulsilver (it was annoying) and caught him without one. it was my friends game. he wasted the masterball catching Lugia, at level 40.

Only Suicune is worthy of a masterball. 1 quick ball, 70 net balls, and 30 ultra balls later. And 15 great balls later.

I agree with Mesprit