The short answer is that current airline seats and seatbelt systems are pretty safe when used in accordance with their design criteria. They are not merely decorative - they are tested and proven safety devices.
The longer answer requires looking at the regulations and understanding what the seat/seatbelt/aircraft interior design as an overall system is intended to do for passengers in the event of an incident.
The current seat belts on FAA-certificated transport-category (airliner) aircraft are governed by FAR 25.785.
I've excerpted a few relevant bits of that regulation below.
Each seat, berth, safety belt, harness, and adjacent part of the airplane at each station designated as occupiable during takeoff and landing must be designed so that a person making proper use of these facilities will not suffer serious injury in an emergency landing as a result of the inertia forces specified in §§25.561 and 25.562.
Straightforward enough: "If we crash people sitting in the seats shouldn't suffer serious injury."
The loads the seats and seat belts must withstand are, in part, described in FAR 25.561 and 25.562 which deal with emergency landing conditions - among the requirements are for the seat and seat belt to handle a 9 G forward load with FAA-Standard 170-pound adults.
The 170-pound adult certification criteria is why you will find discussions about how larger/heavier individuals are "less safe" -- a heavier individual will impose more load on the seats, seat belts, and attachment points, and may exceed the ultimate limit load of an approved seat/seat belt design.
Each occupant of a seat … must be protected from head injury by a safety belt and an energy absorbing rest that will support the arms, shoulders, head, and spine, or by a safety belt and shoulder harness that will prevent the head from contacting any injurious object.
In plain English:
"Either you need to be secured with a shoulder harness so you can't hit anything, or the seat in front of you needs to be soft and padded so it won't crack your skull open and kill you when you're in the brace position."
(This is the same logic used in school busses: Rather than strapping every passenger down with a 5-point restraint we make sure they'll stay in their seat and everything they can hit from a seated position is soft enough that it probably won't kill them.)
Each seat or berth, and its supporting structure, and each safety belt or harness and its anchorage must be designed for an occupant weight of 170 pounds, considering the maximum load factors, inertia forces, and reactions among the occupant, seat, safety belt, and harness for each relevant flight and ground load condition
i.e. "In a worst-case crash a FAA-Standard 170 pound adult shouldn't break the seat or rip the seat belt out of the attachment points." as I described earlier. In practice the regulation sets a minimum standard, and most seats will withstand more force than that.