Group concept

[Dab]

however most factions can buy their own repair ships and use them in clan fights...

Yes they can, but most factions also don't want to put a member in a non-combat ship that cannot fight. Since the people flying these repair ships would not be in your faction, you hire them like mercs, and it effectively gives you a new member for a small time that helps you heal your ship.
Think of a real mercenary. You hire them to bolster the amount of ships you have in a fight. It adds more damage you can do to your enemy and more people your enemy must fight. This concept also increases your numbers, but can potentially be more helpful than a single fight or bomber, as it can give multiple full bot recoveries to cruisers, gunboats, and battleships. Almost 4 full recharges of your bot capacity for a BS.
Imagine two Osirises fighting. One has Stock bots. 700/700. Then the other has a mining ship helping him.. Effectively he has 4700/4700. Now do you think the first BS has any chance? Now if it were an Osiris vs an Osiris and bomber, that first Osiris has a small chance. The first scenario gives him absolutely no chance.
Also, your probably thinking he could get back to even by killing the Repair ship. There are two things wrong with this. One; if he does kill it, the repair ship will have used all its bots/batts staying alive, and that Osiris would still have nothing to pick up. Two; The time it takes to make the repair ship use all 4,000 bots and batts to stay alive, and then finally kill it (remember that it is more agile than you), the other BS would have done so much damage to you, you would not be capable of fighting back.

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