The problem is how you are going to move it around by human means.
And by human means I mean Warp Travel (not Webway).
Think of the huge warp tentacle-thing creatures it would attract. In comparison, voidships are tremendously small in comparison to a Moon and they are completely f-up when they (even small as they are) attract those monsters.
Those things ignore geller fields for the most part being able to deal massive damage even with gellers active. You'd need massive, MASSIVE macro-batteries to be able to push those back while in warp transit.
Also, have in mind that you don't have also a mobile sun while in the warp or when moving to the jump point.
The temperature changes on the planet's surface would cause huge storms and make life impossible without void shields and life-support systems.
So, what's the use of having a mobile planetoid if you will have to basically mount voidships on it anyway everytime you want to expand your base?
I'd much rather invest on having a large fleet. Which will call much less attention from warp "gargants" while being way more flexible.
The reason to have one is the same reason that made me skip the original question: it's space volume is practically game-breaking.
Why get cruiser if you can get a pair of raiders?
Think about warp-capable hive/forge/shrine world it'd be worth of investing massive amounts into voidshields and gellar fields. It's basically battleship going 11 - Human made craftworld indeed.
You won't need massive macrocannons as you can have them is massive amounts - limitless almost.
Life on the surface will be impossible and/or unpractical, but hey - how much space volume you've got under surface?