If one has to reach for the box of obscure, highly unlikely possibilities, then your point is not really going to be valid. We could also say that you are going to play on a planet that is extremely dusty and is going to negate your energy weapons' damage significantly. It would have to be rather significant to reduce a ppc to that of the damage of an ac2.
Precision ammo is nice, but you need to customize existing designs to make any real use for 10s and 20s. 5s and 2s will work, most likely you can swap the ammo out. But, as someone already mentioned, even if they act like pulse lasers, how much damage are you really doing? A mauler could go from "likely to hit with ac2s" to "extremely likely to hit with ac2s", but that is a mere 8 points of damage. Even if using floating crits, which is nice, what are your chances of actually getting? Even still, what are the chances that you are going to ko the unit instead of busting a small laser that was extremely unlikely to ever come into play?
Is the tonnage put into ac2s going to yield enough of a chance of getting a knock out crit to offset the potential firepower gain if the tonnage was put into other types of weapons? If the answer is "yes", then by all means go with the 2s. If the answer is "50/50" then by all means rock what you like and no one can criticize. If the answer is, "well, maybe if the moon aligns with jupiter and the cat rolls over 5 times in the hallway as jack eats the last of the cheetos" then your selection is far from optimal and comes down to nothing but preference.
While I myself never went with what units that were designed to get the most munch out of the tonnage, sub-par weapons remain sub-par weapons in spite of preference. I won't argue that ac2s should not be used, or that anyone design a unit around ac2s to try and show that they can be a real threat, but I won't argue that they are on part with energy weapons for the tonnage.
I really like the Mauler. I really like the Kraken. I like trying to find something that is underused and trying to show someone that it can still be a valid choice, regardless of it being sub-optimal. I just wish that at some point the Devs will make certain things tourney legal level rules instead of optional. There really is the possibility of doing so without breaking the game. As it is, ACs have to great of a drawback in comparison to energy weapons. I can't stand davion, but I would like to see the existing weapon types given more of a distinction instead of it just having another name and slightly different stats. Stats that once the game has started make no true difference.
I would really like to see something along the lines of what we see in mechwarrior games when it comes to weapon cycles. The greater the damage the weapon does, the longer it takes to fire again. Smaller weapons can fire sooner then their larger counter-parts. It could make lower tech games a bit more deadly, which I feel is needed since playing rock'em sock'em robots can get old real fast. More high tech weapons would not be fired twice per turn by the player simply due to the higher levels of heat, which even with double heatsinks could be a serious pain.
An AC2 and 5 could fire twice a turn, much like the rpg rules. A 20 would not be ready to fire again before the end of the turn, but a 10 might. Add a +1 to hit for the second shot and the heat and the lighter cannons could become better though unlikely to truly match their energy based counterparts. Ac using mechs would still be balanced by the heat which can easily threaten to blow them up from the inside. Energy weapons would still have that advantage of being more accurate then a twice fired AC2 or 5, as well as hitting the same location instead of having to roll another location for the second round that hits. An ultra cannon could have the chance to jam(which lets face it should be the problem instead of burning out, right RACs?) increased for each burst. While 4 shots would be a significant change, the heat doubled and the risked increased by 50% in addition to the inaccuracy gain. To add even more to consider is the increased rate of fire's need for ammo. More ammo means greater risks of crits and heat related problems.