Dagannoth fledgelings aren't particularly useful to fight. Although they won't attack players (being too busy with Bardur) and have very low health, they provide no combat or Slayer experience and drop nothing of value. Another downside is that they are located very deep in the Waterbirth Island dungeon, meaning even if they did give remotely lucrative drops or exp, you'd have to go far out of your way for them.
A use for them could be for a player who wishes to avoid Dagannoth tasks without wanting to spend slayer points on cancelling tasks. While they give no slayer exp they count as Dagannoths on slayer task.