Ah, I prefer HW to a solider, but this is what I see which is good about soliders:
He can rocket jump to inaccessable areas, and in combat a rocket jump pretty much grants a couple of seconds of invulnerability (unless there's a good HW around) plus easy targetting. Very good soldiers with a medic can almost fly around a map (watch Wonderwall play some time).
He can kill easily any class in 1-1 combat except a HW at good medium range.
He tends to take and survive a lot of damage, making him a good medic buddy who charges the uber quickly (though not quite as good as HW for absorbing punishment). Plus, he's a decent recipient for the kritzkrieg.
He can close down some snipers with suppressive rocket fire (which, as a HW, I find invaluable; I can't operate if there are distant snipers).
And, you're wrong about the crits. He'll get more crits than most other classes, as the damage spread of rockets builds his crit% chance very efficiently. I think that only a HW crits more often.
Haha, you can see my class bias throughout this, huh?