I am not saying they just crack. In most cases where a Tacoma is rear ended the beds tend to crack. The rubber insert that covers the whole bed might aid in distributing the force if throwing heavy objects in the bed. There is very little reinforcement under the composite bed. I am only sharing my experience. I am pretty sure Toyota did this for gas mileage reasons to drop weight. Non the less me personally for what I do I wish they would have stuck with the steel bed.
The move to an SMC bed likely had many reasons. Weight savings is probably one. Production and transportation costs during manufacturing may also have been factors. I believe all Tundra years have steel beds, so size/strength may be a limiting factor in the use of Toyota's current SMC tech. If someone wants a late model Toyota pickup with a steel bed, it's either a Tundra, or a Tacoma with a flat bed conversion.