You can actually kite many slow ships with a Fortress or Mini Fortress. Just be careful don't get caught by a blob of Bombers. Fortresses heal themselves even if the countdown is on: it just cannot repair other ships.
I use this tactic to clean up a planet with 400-500 threat in supply before I take it. Bring in 40 MkIII engineers and some Mobile Builders in a cloaked transport, and you should be able to put down a Fortress MkI under 10 seconds.
Beachhead if you like, build Spider Turrets if you like, just be careful of Bombers.
Works on 7 Diff Turtle MkIV world.
Notice: Mini Fortresses actually does 10 times more damage to Bombers than regular ones because they don't suffer that 0.01 multiplier. But it is still so pathetic that you'd better use them as a big MkI engineer that doesn't kill itself charging into the enemy trying to fix your Minefiled that fixes your giant 200-turret-blob at once. Treat its DPS as a bonus to its repairing ability: it is under Support tab anyway.
MFs are good if you have a huge combat engineering team like I do. OR when the AI gets Spire Maw. Lots of times my eng's just get eaten by them so I have to escort my Eng team. When I only have MkII engineers, I spare 20 of them into my combat eng team. 40 MkIIIs if I have them unlocked. Therefore, the majority of my planets only get one engineer to build stuff. I put MF's on frontline worlds, to fix turrets. Actually turrets do last longer, and Mini-Forts are seldom targeted by enemy (provided you keep the Bombers far far away).
Last note: A beachhead consisting of two MF, some gravs, some spiders, some Missiles, does not alert adjacent planets unless you put >49 of them there. You still get the one-cap-turret-without-multiplier DPS from Mini Forts, gathers scout intel, annoys the enemy, and not alerting them too much. A regular fortress by itself will alert adjacent planets, though. Of course, that beachhead is not particularly strong. Don't even try that on higher mark worlds. But they do spares you some engineers