Given the importance of a balanced Civ, I think the "barrel straight down the tech tree forsaking all others" problem is handily resolved. Sure, you could research nothing but pure military tech, but your people would be barbarians. They'd be at your throat, protesting your lack of culture. Sure, it would be theoretically possible to build a nuclear missile without developing a written language, but you'd be a sad, impotent nuclear power. One whose instructions have to be memorized orally, too. ("Hey, Frank, are we supposed to hit the red button or the blue one to put the missile into storage?" "I don't remember. Lemme check my-- oh, wait, I don't have an instruction manual, since we can't read." "Ah, let's just flip a coin." "But we haven't developed currency yet!")
Conversely, you could build a massive supercomputer, but you'd be unable to defend your cities with anything other than rock-wielding primitives. The illiteral nuclear power above would march into your cities with impunity, then discover that, having no culture, he can't build so much as a keychain in any of his cities since the people riot too much.
The above two cases will occur rarely, partly because neither Civ would last very long, but mostly due to trade. If the nuclear power in A and the rock-wielding supergeniuses in B were to meet early on, B would teach A how to read and write, while A would teach B the value of bronze-tipped spears. Thus, you can have one psychotic warmonger and one hippy culture freak, and the two can share their knowlege to live in harmony. Or the warmonger can beat the hippy until the hippy tells him everything he knows, then kill the culture freak. But that's known as bullying, and it's not very nice.
