Buzu,
AMM's are not foolproof, and I have played games in which the AI effectively counters them with Corvettes, Phoenixes and even (AF-assisted) Scouts.
First, their low DF, combined with a minimal # of HP's & slow speed, renders them vulnerable to any attacker possessing a speed bonus. Mid-game, it is not uncommon for an AI to have a 1-2 MF advantage over them (esp. with Gravity Accelerators) and be able to position a far less expensive "throwaway" screen within range to either (a) take them out, or (b) force you to expend one or more of them against it.
In a longer-term game (on Large/Huge maps), you are effectively conceding an IU advantage of 10-50 points per vessel lost when exchanging an AMM for a Corvette, Phoenix or Scout. AMM's are only cost-effective if sent against capital ships (BC's or above). This presumes there is no screen that either does not attack first, or only has 1-2 MF with which to attack a cluster of AMM's (and so can only eliminate 1 or 2).
A favorite AI tactic under 1.03.057A is to send a light vessel adjacent to any planet and attack any AMM's it encounters in orbit. The AI is quite content to exchange these ships for AMM's whenever possible.
Secondly, AMM's by nature cannot gain experience - they are one-shot deals. This is important when one considers that each level of experience is equal to +1 AF/DF (a 2 pt. gain total).
Thirdly, even with a 100+ AF, AMM's can misfire and fail to kill their target. I have seen them leave even BC's with 2 or 3 HP's left. It is not common, but it happens enough to rule out making AMM's your only line of defense. You need capital ships, with multiple MF's, to "finish off" crippled targets (or expend a second AMM to kill something with only that # of HP's).
Lastly, it is common mid- to late-game for the AI to have built it's "ship-bonus" structures (Shipyards, SB modules, etc.), raising its ship values. This necessitates expending multiple AMM's to eliminate vessels, driving the IU ratio ("kill-cost") up even higher. At 600-900 IU's (2-3 AMM's), only Rangers, Avatars & Overlords are cost-effective targets.
Thus, they are a useful tool (and vital for mid-game survival), but not a panacea.
I too, however, have never seen the AI build them. The only time I have seen the AI use them is when they are given as a gift, in which case the AI sets them to auto-attack the nearest enemy ship, regardless of size. (Very dissapointing to watch an AMM you just gave your closest ally in trouble go directly after a scout...). This might be why they don't bother to build them.
~SDC~