That decision was made by the developers of Factorio for the base game, not the developer of IR3 for this mod. And the rocket control unit has a different recipe in IR3, so practically it’s not even the same item. The reasons for its removal from vanilla could not be applied here. IR3 also has a second “rocket only” item, the hull section. I think it’s clear that’s just how the mod approaches rocket parts, and since this is only a pseudo-port of sorts, I think it should not make any changes to the mod’s design, and aim to enable a faithful experience.

You're right, I don't really have a good counterargument to this. The reason I removed it in the first place was just to make life easier on me, while I was still working hard just to get IR3 to load to the main menu in 2.0.

I also wanted to make a code-only patchset that didn't contain any assets. But the rocket control unit is a base-game asset, so it should be fine to include.

Now that the Patchset is beginning to stabilise, I'll put this in the next update. It's technically a breaking change, but you all saw the ALPHA sign on the way in!

EDIT: There are some other changes in the Patchset as well, where I've generally followed the rule of "This is something that I think Deadlock would have done for a 2.0 update". Removing filter inserters is a good example of this.