Not to mention if you build mix units you don't have a strong point, your more of a jack of all trades, and you know what they say about jack.... He's not really great at any one of those trades.

You wanted an example. Say your OP munching, You could build a sqaud of armoured infantry. For Out-of-Protection you could build anti-inf/veh vehicles, or tanks even. but bottom line is that you want to mostly concentrate on one type. It will serve you better in time of war. Other members of your alliance would concentrate on another type and so on.....