Thanks for asking. I worry that it could cut both ways. Here are some hypotheticals:
As a rookie, especially as a once-in-a-while- and I-met-them-at-a-GTG-and-they-seem-great- basis, how do you officially set their expectation on how they can use their shots, based on the contract you have with the client?
Are there restrictions on you that must be passed along to them? Confidentiality (business location/product shoot)?
If you have them assist with workflow, or have access to your shots.
Can they take the shots (appropriate credits or not) and jazz them up for their own portfolio?
Do you insist (and check to see if) they have their own liability?
Intern arrangements are much more supervised and are (I would think) easier to set and reset expectations...but wouldn't those experiences on the photog underscore the need for a contract with any off-the-street second shooter.
I would not want to encumber a professional arrangement with a long legal contract, but what seems appropriate...I always hate to see the relationship end on a sour note when a reasonable framework can be put in place. To put in a way that other threads have, perhaps,

"lay down the law, and nobody gets dished at, blackballed, banished...", and all the rest of the vitriol.