Frequent Guest as Co-Tenant?

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I just read "NOLO Every Landlord's Legal guide".

I'm confused with this scenario:

Let's say the Property is located in a very desirable location in California, very easy to find tenants.

Tenant [T] : has 750 credit score, she has clean records.

1-month later, [T] want his new boyfriend [B] to stay as her roommate (also to save money on her rent).

As landlord, is it really important to add [B] on the lease as co-tenant?

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For me, it seems like it has more benefit if the [B]oyfriend stay as 'Guest' instead of co-tenant:

+ T is still obligated to pay rent to landlord anyway.

+ T retains her power as sole-tenant, can kick [B] at anytime (to settle disputes faster).

+ Anything happen to the Apt, since [B] is T's guest, will [T]'s renter-insurance cover the damages?

+ If anything serious happened, can't Landlord just evict everyone since technically [T] breached the lease?

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So if [T] is willing, and most likely can handle payment & liabilities, isn't it a lot less hassle just to turn a blind eye and let [B] as guest?

What's the possible legal issue that can arise in the future if landlord keep turning a blind eye, not adding frequent Guest as co-tenant?