Actually, they can.
What I mean is if you are using a five cell receiver pack, but you used to use a four cell AA battery holder, with rechargable batteries inside, then, you would be adding an additional 1.2 volts to the opperation of your reciever and servos.
you also save weight because a typical rc specific battery, that uses 2/3A type batteries, weighs less than that battery cradle with four AA sized batteries in place.
But for some cars, an RC specific battery is necesarry, and these gas cars would be 1/8 onroad cars, kyosho voner, Trinity nitro Reflex, Mugen MTX2 and Yokomo GTR4 unless you could find that funky battery box that they use in the RTR version of the car. There are many other cars that require the use of rc specific batteries. i say its better to get now what you will need later, that way you don't panick when later comes.
*"I see" said the blind man to the deaf woman*