Not only that but unless you're using system smokes (secondary) with integral sounders the only sound you'll get is from your notification device(s) which wont sound anything like a line voltage smoke.
Adding secondary system smokes are nothing but a benefit for the client - and I will always recommend it!
On another note: Michigan for example wont allow just system smokes (residentially) for a interesting reason - dealers shut down (lock out or disable) systems (burglary, hybrid, fire etc) and some have left their ex customers with no fire protection, apparently a big issue there...
I personally charge for my service and equipment whilst installing, so the customer doesn't owe me for equipment. I feel it's criminal to shut down a "paid for" fire system because the client hasn't paid their monitoring contract or has breeched contract. Not only criminal but down right wrong and insanely ridiculous from a liability standpoint - I don't care how good your contract is...
Jim