Should all housemates be on a lease? It depends.
Yes, everyone on the lease has a shared responsibility towards the landlord for rent and the care of the home.
No, everyone is locked into the arrangement for a definite period of time. If the arrangement isn’t working, it’s hard to change it.
There is a good discussion of the legal aspects of sharing an apartment in NYC by Lisa Stellin, “How to Avoid Roommate Regret” in the NY Times.
Leases don’t make relationships work. Sharing housing works when there is agreement on the essential ways that housemates must cooperate. Most importantly, these agreements must be made before moving in.
More reading here and lots more info “Sharing Housing, A Guidebook for Finding and Keeping Good Housemates”
After writing this post, The Common Law: Cash Rental Payments and Flaky Roommates showed up in the Austin Chronicle.