Don't underestimate this dino's usefulness. It may not do damage but it's knock back is very useful for holding off raiders. If your base is near a cliff you can knock players and dinos off it. Another thing is that diplos actually do trample damage. This means that it can kill small creatures like bugs, dodos, and compies by walking in them. In addition there is no level requisite to tame it, so it colud be the first dino you tame.
Not as useless as you might think! A high-level spino was terrorizing a nearby river. I took my diplo (Dylan) and while the spino was focused on killing Dylan, I was using my crossbow and rifle to do damage to it. Spino ended up dead, while Dylan was at 2000 out of 3000 health, and I was not injured at all.
Love u Dylan