The old A-Wing Runaway Tactic always brings up the same old questions.
Legal? Yes. Nothing in the rules to prevent it.
Viable tactic? Yes, because it's the classic hit and run tactic that has been used in most forms of combat since Thrug threw the first rock at Grug.
Poor sportsmanship? Depends on your point of view. When a player throws out the accusation of poor sportsmanship, they need to have some solid grounds to back it up. If you got beaten by an opponent with superior agility, why is it considered that they didn't play in a sporting manner? The object of any combat game is to inflict damage on the enemy, while minimising your own.
It's been said here that the "poor sportsmanship" cry is usually used by those that disagree with the way some players do things, even though they are within the rules. I see it as a pretty large calibre accusation that falls just short of cheating. It's one thing to sit across the table from the A-Wing Runaway and tell your opponent "that's a pretty crummy tactic", but it's a whole lot more confrontational to tell him "that's just poor sportsmanship".
Would I play the A-Wing Runaway? No, it's just not my style. I play for fun, and the tactic of avoiding combat doesn't seem like fun to me.