There is no problem with having the light on the same circuit.

In fact it is probably better that the light is on the circuit-
It will be noticed if the light does not work, and possibly be a tripped breaker.
With only the smokes on the circuit, if the breaker trips, you may never know!

Many codes require the smoke detectors to be on a circuit with important lighting fixtures so that one would not turn off the circuit and leave it that way. Rather than the NEC, these codes are building or safety codes.

With the smoke alarms required today you should know if the ac power is lost because the battery backup will beep to let you know if either the ac or battery power is lost it will beep.

Right, but, say you're gone on vacation and the circuit goes out, beeping starts and wears out the batteries. You come home and the circuit is out and batteries are dead and you don't know it.