Fleeing units still scatter into neighboring friendly provinces, alas, even if their army wins the battle. (I guess they're too chicken to come back to the camp right away and get laughed at.
) If their flight is interrupted by victory (in other words, if they don't exit the map before you win) then they're still with the army after the battle.
The benefit I've been finding with Fire and Flee is that they'll fire away for 3 turns, but then retreat before the enemy can close ranks and whomp them back. It works especially well against infantry, obviously, including those with the shorter-ranged javelins. I don't use it all that often, for the exact reason you cited: it is dull having to round them up again afterwords. Still, even though I can't recommend it as a universal tactic, I've found it useful, especially in the early game where the low income situation means rounding them back up is a lot less costly, relatively speaking, than replacing them. In that early game, they will often end up having only one place to retreat to anyway.