Infantry, if they ever had ANY value, might be in seizeing an opponents colony (and it'd probobly take an assload of them, as you'd be fighting their entire population). Out in the open, infantry are extremely vulnerable, especially on a planet like New Terra. We can afford to throw people's lives away in infantry right now, theres about 6 billion of em to sacrifice. But when you have only a small population, every last person counts, and infantry is a useless waste of lives, when tanks do the same job just as well.