Yea, they aren't needed but they can be really helpful.
Like one time, I was trying to take out a wraith guard post (pre-nerf), and I didn't have any cloaking ships on me. Well, I found a Mk. V space plane manufactory, and used that to help me take it out.
Could I have won without it? Yes, as when I was "bashing my head" into it with my fleet ball, I was accumulating damage each time, enough to overcome the auto-regen, even given rebuild times.
However, doing it that way was very slow and somewhat boring, so getting that fabricator opened up a new possibility that let me do something much more quickly and cost efficiently, not to mention less grindy.
That said, when you are properly managing AI buildup, you generally don't need more than Mk. II to win, even if you go up to "mid AIP" (around 250) and take no superweapons. It just takes longer (or maybe shorter, depending on how much the AI grows each AIP point and the map layout) (as mentioned, only if both AIs are <9)
Maybe this is stemming back to the feeling some have the AIP is currently TOO restrictive right now even at the "balance point" levels (6-8.x), and needs a "nerf". (I still do contend that the exponential scaling of the AI >=8.3 or whatever is indeed too restrictive. I would push for polynomial growth at these difficulties instead)