Although your commander is a powerful unit, it is inconveniently a slow unit. Plus, you have only one commander available until your economy is powerful enough to build support commanders.
So the alternative is, of course, engineers. But few players realize how important many, many engineers can be. Building thousands of engineers is just plain obsessive and useless, but building only 8-12 engineers could be one of the things that causes a player to lose a game.
It's important to have engineers everywhere. Engineers inside your base will help speed up production, increase your economy and build your necessary defenses, among other things. Engineers outside your base are great for expansion and for collecting excess mass. You can set them to patrol sections of the map to automatically move around and collect metal scraps, trees, and rocks instead of manually right-clicking each scrap you see on the map. You'll be surprised to find that they will automatically reclaim small pieces that only look like a pile of soot, but can a large of chunk of mass that you wouldn't have clicked otherwise.
It is worth repeating that it is important to have a vast number of engineers out in the field. The best way to do this is to include one or two engineers in your build list for factories. Then, when you send out your attack force, send your engineers close behind -- unfortunately, few players do this.
Many players choose not to send engineers with their attack force because they don't want to lose them to the opposing team's army. It is true that engineers are fragile and have no weapons, and so will die with your army if you lose a battle. But, you lose no advantage by losing extra engineers. You do gain an advantage over your opponent if even one of those engineers survives. Because now, instead of having to send an engineer all the way from your base to do what you need to, you have that engineer already there. Now you can repair units, replace buildings that the opposing building destroyed, collect the mass/energy of the destroyed units from the battle (as well as trees and rocks) and build radar, defense, or whatever you desire to build--all instantaneously, rather than having to wait for an engineer to arrive.
During the middle-game, Support Commanders can also be used to collect metal scraps in groups as they have much more HP than a Tech 3 engineer and can fight back against T1 and T2 units at a decent rate. If you have a group of 5 or more, a few T3 units don't pose much of an emergency either. They can usually stick the battle out, or retreat in time without the risk of being killed. Their unique-to-faction upgrade features can also make them much more self-reliant while patrolling the battle aftermath for scraps of metal. Support Commanders prior to upgrades have a build rate of 4 times faster than T3 engineers, making them very quick at reclaiming wreckage. If you have many patrolling an area, it is wise to have some T1 mass storage units around your base so their collection potential isn't wasted.
Special note should be made of the UEF T2 Sparky combat engineer, an engineer explicitly designed for battlefield use. It is armed with a Hell's Fury Riot Gun, has sensor and jammer suites, has more HP than a normal T2 engineer and moves faster than any other engineering unit except the Rover drone. Its drawback is that it is restricted only to building attack and defence structures, but it can capture and reclaim just as well as a normal engineer and if another T2 engineer starts building a non-combat structure for it, it can assist as well.