@ChrisVJ: these balance weights and balance shaft all have their weight, too, and MUST have their weight to be any good. Here comes in the natural superiority of the boxer engine, which doesn't need all these additions to run smoothly.
As for needing a gearbox: yes, modern diesels develop maximum torque at very low revs - the 3,0 Iveco in my van even at 1400 rpm! - but, though torque is important, we still want power at the prop, and power is torque multiplied by rpm. Revving up the engine will still bring improved performance. Whether a gearbox brings enough improvement to justify the added weight/complexity/maintenance is any designer's decision.