I think this is a bad, bad, bad idea.
90% of the time it is better for the AI to NOT have their ships in fleets in orbit.
Except the AI *does* leave loads of ships in orbit. So this simple rules change, which is good for a bunch of other reasons as well (including just plain making sense), also has the effect of making the AI more competitive.
The fact is that without a *major* brain transplant, the AI sucks at manouver. Truly. It is just *not smart enough* to play an effective game of intercepting attackers, so it probably should leave some portion of its military sitting on its worlds to prevent players from simply seizing them all with very high speed transports and such.
I just played a game in which the Terrans were a pretty powerful neighbor and declared war on me. They were rated as #2 military in the galaxy. I flew around and started picking off the defenders around all their worlds, picking off their fleets flying around... and their military rating was still high. My transports took Earth itself. Why was their military rating still so high, I wondered? Then some of my cruisers went on a scouting mission toward one lonely Terran planet sitting way off in the upper right corner of the screen. They ended up in the middle of a HUGE armada. Battleship after cruiser after battleship. Seeing my ships seemed to trigger this armada, which promptly started to swarm all over the place. It took some real effort for me to take it all out. If the Terrans had that thing defending their worlds, or had thrown it at me in an attack or counterattack, I would have been in *trouble*. But it just sat in deep space twiddling its thumbs.
Turns out the far-off planet had some incredible bonus tiles that set it up as a *very* sweet manufacturing world. The Terrans were probably building most of their fleet there. But for some reason they just sort of collected it in space. Well *away* from their planets, but in what some piece of code might have decided was a geographically central part of their empire.
I've never had trouble outmanouvering the AI. Sometimes you can even just use a cheap decoy ship to lure them away.
Like it or not, the more this game encourages tricky manouvers the more the AI is screwed.
In my opinion the problem for the AI is when they build a fleet manager because that makes it easier to take out their ships in fewer turns.
I can't think of when it'd matter whether I can take out the AI's ships in one turn versus two or three. On the other hand, there ARE cases when the AI will have several ships that could easily be picked off by one of my raiding parties (free experience!) but that would actually take out at least one of my attackers, or even require me to bring in a full-scale assault fleet, if they could fleet up.
It's not uncommon to have an AI declare war on me when I'm not actually ready for their total and brutal annihlation. In that case, a cheap and easy way to keep them down is to send raiding parties to pick off the non-fleeted ships defending their worlds.