I played a 2v2v2v2 game on hard. I was vasari and my computer ally was advent. So, as my species progressed across the galaxy, my ally started changing his fleet. (By this time I was using the dark armada fleet, so it gave me whatever random ships it wanted to, no siege ships - probably because i never bothered to research them). My ally must have picked up on my complete lack of siege ships because he had a TON of them. By a ton, I mean about 75% of his almost maxed out fleet capacity. I wish I had a replay to show you, it was truly hilarious. I would raze a fleet outside an enemy planet, and skip to the next while my ally came right behind me with countless siege frigates and wasted the planet. At the end of the game, the ally had actually managed to secure more planets than I had by doing this!
I've never fought against an AI enemy that had so many siege frigates, but it is a possibility that it was a component to his own ally. IE, like my ally complemented my weakness.
I did however fight against a TEC enemy in a 1v1 game that used nothing but carriers and repair ships with his cap ships. Utterly wasted me as I had no anti-air defense except hangers, which his fighters took down fairly quickly.