...and they can be upgraded later.

Note that I did not praise the AI tactic, just that there is a basis for the script that directs it.

In addition, forcing the expense of the addition of weapons onto invasion transports (that go away), has some value, as well.

I would also note that (at least in DA) some AIs put weapons on transports, perhaps anticipating " 'neutered' " defenders.

Speaking of said defenceless ships, I build them early in the game so when I do get weapons tech, I can just upgrade them.

Poof: instant military!

Building hulls and doing an upgrade is much faster than building real ships, yes.  Provided your ships are within range of a friendly base/planet, they'll only take until the next week (that is, turn) to upgrade-as close to instant as GC2 gets.

Building hulls and upgrading them is usually cheaper than buying outright-but it's still quite expensive compared to building it with production.

Since the function for base rush buy cost is trunc(cost^1.1)*6, this means you're paying anywhere from about 8x on the low end to as much as nearly 14x by the time you get to a ~4000BC production cost ship (which has a no-lease rush buy cost of 55002BC).

Upgrades seem to be a little more complicated, particularly when involving troop modules, but the basic premise remains true.