Because not all of the roommates are on the lease, the liability for
rent and other obligations may not be shared equally. You could be
held liable for your roommates' rent defaults and damages.
With roommates, even if everyone has signed the lease, you may be responsible for the entire lease liability if any roommate defaults. If you have any concern about a roommate potentially not paying the rent, you should consider the consequences prior to a default.
Regardless of who has contributed to the security deposit, you may
be at risk for any contribution to, or be liable to the landlord for, damage
you didn't cause.
Shared expenses can get tricky. A roommate agreement will
help you set the ground rules for who owes and who pays.
Without well-defined house rules, great relationships can be ruined.
A roommate agreement lets you and your roommates decide and
set the rules of the road. With input and agreement from everyone,
things can run more smoothly.
With frequent disagreements, it is always better to have things in writing. It can avoid many disagreements or protect you in the event that things blow up.