[Ulzgoroth]

Interestingly, it's much easier to justify a rider-and-carrier model with reaction engines than with reactionless.

A carrier, as a long-range vessel, needs to pack long-term accommodation and high-delta-V drives, plus possibly FTL drives and big reaction mass tanks. A rider wants higher-thrust drives, and by off-loading the long-range drive and support elements to the carrier it gets to reduce the mass it has to push around with its much more expensive engines.

If the rider engines are rockets, that basically falls out directly. If the rider engines are reactionless, you've got to go out of your way with something like weby's system to explain why the carriers don't use those same engines. (Riders with the same core thrusters as their carriers are justifiable, but have a lot less going for them.)