Not really any reason not to, but it depends on what you're trying to accomplish.

-Doubt it will have any effect on noise, unless you're used to riding in the bed of the truck.
-New and later model trucks have zinc coating then painted beds, inside/underside, will most likely last well beyond original purchaser's use.
Unless you really abuse it big time.
-Sprayed on coatings will still only cover so much. Will not prevent flow of materials (water, dirt, rocks, etc) from being thrown up and around the cavities between the fender and bottom of bed, or to the backside of truck behind bumper.
-But spray on will protect what ever surface it is sprayed on, just a matter of whether it's a worthwhile investment is up to you????