...unless you are taking over the appartment from them ('Untermieter') and have a contract with them rather than the landlord.
I've done this but only with someone I knew and trusted. The only reason why they may want this set-up is if they're not telling the landlord you two will be moving in. It may be in your interest in terms of getting the place but as others have mentioned it's not ideal and could leave you holding the baby.
Landlords are generally easy about others taking over once the old tenants have found replacements and they'd probably be happier knowing your name was on the deposit account.
Also, paying huge amounts of cash as deposit is sadly still the norm but you can use the insurance policy offered by some associations where you pay around CHF 200 pa instead of locking in your cash. Search similar threads for links (sorry, can't remember them now

).