, I have found a great use for them as a defensive force. An army of 4 vetran infantry is much more defendable than 4 separate units, especially when accompanying an offensive force.
As an example, you have 4 tanks and 4 infantry (unfortified) and the AI attacks with 8 tanks. Each of the first 4 AI tanks die and reduce each of your 4 infantry from 4 to 2 hp. The next 4 tanks attack and your tanks defends, because they are stronger (4hp tank vs. 2hp infantry), and all 4 of your tanks are killed. Consider the same situation with 4 infantry in an army. After the first 4 tanks attack, your army is down from 16 hp to 8hp and is still the strongest defender. For the next wave of attacking tanks, your (weakened) army is still the target, and your tanks remain untouched and ready to strike the following turn. Though the math presented here is not exact, the basic premise should be clear, and the princple sound.
I began using this strategy to defend an amphibious landing force, but soon I began to use it accompanying spearheading forces invading enemy territory. Two infantry armies once defended an onslaught of over 75 cavalry allowing the loss of a single offensive unit! More recently, I have applied the same ideas in the middle ages and ancient times with amazing success. An added bonus is the effect garrisoning an army of infantry (or rifleman or whatever) has on resistors in a recently captured city.