I understand the weaponless ships from a planet protection standpoint bu tthe upgrading is hard to justify. As many scouts as the AI builds it would seem they could be programmed to park them as planet protectors.

Damned if you do or don't I guess. If you add value to defense the AI builds it but they don't understand it is all but worthless without attack. Can the AI be programmed to understand this? Beats me.

The problem is that they are building ships with only defenses when they could have built a ship with nothing on it for far less money and time, since both are just as effective in stopping an early invasion (which I guess that they are for).
That they build more then one weaponless defender on a planet is also pointless. If the enemy has a ship with weapons, the number of weaponless defenders won't make any difference.

The easiest solution would IMO be to forbid the AI from building weaponless defenders completly and have it research weapons earlier if needed.

This is very common in DL campaign.
But in sandbox I've seen only Terrans use it from time to time.

On the other hand attack is cheaper so they've could built more attack ships for same time.

i've seen the drengin to this is a sandbox game.

they build a massive fleet of 0/0 ships, and start putting money into the treasury. when the arceans and their beam weapons rolled in, they upgraded all their ships to rather frightening missile/shield ships, and their military power skyrocketed in a few turns. this was on crippling. in a few short turns, they managed to tear down the arcean military advantage (they had like 3 military resources, which i quickly grabbed up).