We probably dont have him with us because he is simply not strong enough. Having the echo does not make one stronger (only if you are a Garlean somehow..). Otherwise Krile or Minfilia would have been strong too. All the echo ever did (before SB..) was protecting us from being tempered. At least if the WoL goes there alone (or with 7 others if you want to see it that way) they do not have to worry about people like that.

(Who knows maybe all this talk about great fights is all just a way to make others love us and in reality we are just playing some TT against them and are winning all the time °L°)