The rotor tips, or apex seals, should be changed in a rotary engine from time to time. Even with regular maintenance, they sometimes degrade too far at a moment's notice. Rotary engines that are pushing a lot of boost are notorious for blowing their apex seals, causing catastrophic engine failure.
BTW Sinatra, the 13B-rew SHORTblock weighs a mere 125lbs. The longblock with all accessories (including both turbos) weighs over 400lbs. Don't believe me? Look it up.
As for the triple rotor, such as the 20B, that motor wasn't put into the FD3S RX7 for production cost and weight distribution reasons. The engineers responsible felt the car would be nose heavy.