Oddly enough, at this Saturday's league event we had a guy play this exact team (3 x-wing, 2 y-wing = 99 points) and he ended up coming in third place of the 8 players. I think a lot of this was due to this being an odd week where we only had 1 players bring an Imperial team, so he was able to get points leads on a lot of 3-ship rebel teams.
The first volley took down shields and maybe hurt a ship, but didn't kill it (luck possibly). He was then able to counter attack at his lower pilot skill turn and with 5 attacks able to typically take out one of the three targets. He always quickly lost his damaged ship the next turn, but his opponent used up most or all of his attacks to accomplish that, so his remaining 4 ships put a sizable dent in one of the other 2 remaining targets.
This is how he won his two games. Unfortunately I didn't get to play against him, but I hear the team that beat him was 4 ship Rebel team (named X-wing, 2 generic X's, Gold w/ ion).
I guess what I am trying to say is that the 5-ship team can work, but you are betting that you will be able to win the battle of attrition as losses don't mean as much to you as other Rebel builds. It didn't face the Imperial player.
Note: this was an unusual week for us as a lot of regular Imperial players all decided to run 3-ship Rebel builds for a change.