Well when Im playing multiplayer Im usually always pushing somewhere with my dudes, so they are either pushing to take more of the battlefield, or they are dying, hopefully when they do die, I have something better to replace them with. Thats pretty much the way the game paces, theres no research branching as the game is simplified in that way, your just upgrading your stage so your army becomes more powerful than theres, hopefully quicker than there's does. The better units, at least with Ironheade's units, have a good amount of anti-infantry, once you can make those, you need to combine the anti infantry with anti vehicle. So yea, if you let your brutes sit around too long taking up your army space, you should get ride of them in favor of the better stuff if its available.