To be honest I'm not entirely sure how the algorithm for repairing works in PA, but from I've experimented it seems pretty clear that the more metal cost something has, the more it costs to repair. This seems fine on paper, but ingame it makes some really stupid things happen. For example, the Angel is one of my favourite units, but I can't use it. Not because it is overpriced and can't even reclaim (which it is) but because how badly it works. If you ever tried to use, say, 3 angels at once, you will discover two things: 1-Having more than 1 sucks, because they all shoot at the same missile at the same time and overkill it. They waste a lot of shots, fire more than necessary and lag the hell out of your computer while doing so. I wouldn't mind it, but 2-You have to turn them off so they don't repair. Angel has a buildpower of 120 metal/s. That's the highest in the game by a lot. You can't possibly afford to have it on and near a battle, because it'll start repairing **** that will die anyway, possibly overkilling with its repairs (which I'm not sure but seems like they do. Specially if you have more than 1 unit that repairs) The metal cost for repairing it simply too high and happens too fast to be sustained by your economy. It's very situational. Also, repairing high cost units (which should be the primary use of repair) is a nightmare. Repairing the comm takes thousands upon thousands of metal. It shouldn't be too easy to do, but right now is absurdly expensive. So, my conclussions: 1-Repairing atm is very bad unless you're repairing very high hp units with low cost, like turrets or orcas. Otherwise it's a total waste of money you'd better spent producing. 2-Trying to repair with high tier units / a lot of them basically destroys your economy in a blink. 3-Repair cost should depend on the amount of HP you're healing, not the metal cost of the unit. 4-If you're repairing it usually it's because you want a) not waste metal producing the same units again b) not give your opponent scrap, so it makes more sense to have repairing cost more energy and less metal.
In fact, many units have higher hp than its metal price. Wall: 50 metal, 2000 HP Inferno: 225 metal, 1000 HP Ants/ Bolo: 150 metal, 225 HP Leveler: 600 metal, 1500 HP Vanguard: 1500 metal, 5000 HP Ares: 30000 metal, 80000 HP Atlas: 30000 metal. 40000 HP .... But there is also the bad side. Commander: 25000 metal, 12500 hp Dox: 45 metal, 40 hp Slammer: 450 metal, 400hp Zeus: 30000 metal, 10000 hp As you say, they are difficult to repair. To repair or not, it highly depends on the units of your force. In my opinion, the repair isn't that bad. The energy per metal of repairing is much lower than rebuilding a new one since the energy efficiency of repair unit is even better than advance factories and advance fabricators. Repairing can help your units to push farther, especially in big planet. Ships and T2 tanks go quite slowly, and it takes a lot of time for new units to go to the front line. Repairing not only save energy, but also save your time for traveling. About the angles, they are expensive and few, and they should be micro to repair your Titan! If you try to use the hot key to select the air units, you can find out the selection doesn't select your angles. I think it indicates the angle isn't a regular air unit which should fly into the battlefield. You can say the Stitch is more cost effective, but the angle is more convenient. In the late game, the metal income may not be a major problem. So you can start to use some expensive units to reduce your micro, though it may be not the best cost effective. Similar situation happens on the T2 combat fabricator Mend. It takes 1200 metal, but only provides the construction ability of 60 metal/sec. Unlike the T1 combat fabricator bots, Mend isn't get selected when I use the hot key to select the land units. And its repair range is quite long. I guess it is designed to stay at home for repair the nearby air units or defensive structure. Repairing should not kill your metal income. If it hurts your eco, you may build them too many or too early. The angle is the late game units, don't even use them in the middle game. By the way, you also detect the mine of the enemy by Stitch, Mend, and Barnacle. Otherwise, you need to use the skitter.
