Good question, I would like to know as well.
Some people say muskets are faster which would make sense helping the AA build, but some say muskets are not faster which makes rank 7 an open slot.
Though one possible reason might be the cds, since Musketeer has lengthy cd’s the running kneeling shot combo effectively minimizes any down time. Then again the Cannoneer could use their AoE non stop until they need to go into AA mode for single targets. So many choices to make lol…
Edit: How well do cannoneer’s solo AoE mobs since their skills force them to pause a moment?