Well Litanies of Hate and Fearless are good reasons to attach him to the squad. What I've seen happen is the Chaplian joins one squad, assualts with the squad and wipes out a unit. In his next turn he leaves that unit, joins another unit, and assualts with them into another unit. Moving 12" allows them to jump around to join whatever squad needs him the most.Yep.
The only restriction regarding bikes is that if the chaplain has a retinue he may not be mounted on a bike.
But, as mentioned, no transports and he has top move 6" when joined up. Might be best to leave him unattached and just *near* the squad. There's not really any advantage to having him join the squad since ICs can;t be shot at if there's a closer target
But it is a good tactic to not have him joined to the squad prior to your turn so that if some one say "lashes" the unit and assualts it then the chaplian isnt lost with that unit as well. This way the chaplian stays viable to join a second unit to give you an excellent counter assualt.