Since the repair arm seems to always fix one damage (?), why not have it cost some fixed number of scrap rather than a percent? Also it should not cost any scrap if the ship has no damage to repair. I think this would make it more appealing since it isn't getting much love.
We didn't want it to be always a positive, obvious choice. The power of constant repairs after every fight is too useful without at least a few setbacks.
I'm with Gomisan on that I get it whenever it's available. However (I'd have to check the math), I might sell it come sector 6 or 7 if I'm doing well since that 15% starts to hurt a lot more and the extra boost to finish off the boss is helpful.