What is the use of the Shaw-Fujiwaka Translight Engine. The harbour would be too big to pilot through Slip space without ending up half in of a large mass. also the speed at which it travels would leave it far behind anything else.
After the Battle of Harvest, the UNSC decided to retrofit some of their orbital stations with Slipspace drives to aid the evacuation of civilians via orbital elevator. The Harbour-class was one of the first repair station purpose-built to embrace this concept. This allowed them to jump civilians to safety while the UNSC fleet slowed the covenant advance.
Harbour-class vessels can also transition with a fleet to aid combat repairs. They can even 'ferry' smaller vessels with damaged drive engines into action with a fleet, while conducting repairs on them in Slipspace.
Too big? These things are nearly the size of a Marathon. If an Assault Carrier twice it's size can make a Slipspace transition, so can it. It does not need speed to travel far...just to travel to safety....