Short range for Clan tech is really 7 hexes, not 3.
Mounted on a sufficiently mobile mech, you're probably not going to be able to really hang out in hexes 4 to 7. And as previously stated, unless the modifier is extreme, I really don't have a problem firing HE loads out to medium range, so your real sweet spot is exactly 1 hex wide. That's depending a lot on having more mobile Mechs than I do (rare with big box missile boats) and maintaining Initiative at all times.
Your ATM-12 also weighs 10 tons with 3 tons of that for ammo. For the same weight, I can fit in two LRM-15s with 3 tons of ammo, -plus- I don't have to worry about specific types of that 3 tons of ammo being useless at extreme range.
Okay. I'm going to clarify this, so that you can quit beating the dead horse.
If I have ATMs, and you have LRMs, I am not going to shoot you at long range. It's not going to happen. If anything I will be closing as rapidly as possible, using terrain to drive your modifier up as much as possible.
When I run ATM launchers, I include mostly HE ammo with at most one or two tons of ER or Standard ammo.
If I wanted something for short range, I'd take ERPPCs and UAC/20s (but aside from the point, I guess).
And ATMs beat the crap out of the first option in the heat department and the same applies to the second in the weight department. All three have very similar damage profiles but vary in how damage is applied, all three of which have legitimate and tactically useful applications.
Same thing again. For the same tonnage as 4 ATM-6 launchers with 4 tons of ammo (5 salvos of ER and 5 salvos of HE), I've got 3 LRM-20s with 3 tons of ammo (6 salvos) and maximum ammo effectiveness all the way to 7 hexes, so I don't have to pack more ammo than I need.
At 7 hexes you're either wasting your space with Standard or ER ATM ammo or you're whiffing like mad.
Let's challenge that assumption, shall we? We'll use standard ammo for comparison. Medium range 10, so I get a plus 2. We'll go ahead and calculate my pilot versus your pilot, and assume that both pilots are 3/4. I'm at medium range and in a 5/8 mech, walking. That puts me at a base modifier before your movement and any intervening terrain at 6. That's not really all that bad. If you have a similar movement profile and are attempting to keep me at range, you're either making facing changes or backing up, one or the other. If you're making facing changes, you're running, so you're automatically at a 7 to hit (3 base + 2 for run +2 for the fact that I walked 5 hexes).
If you're backing up, then you're moving a limited number of hexes and my base to hit modifer doesn't go up all that much, and you're going to have a harder time keeping me from bringing HE ammo in to play.
Additionally, I get more to hit rolls with the 4 launchers than you do with the 3, placing the law of averages firmly on my side. Beyond that, as previously shown it's a lot easier for me to get my full complement of missiles to hit. Assuming an 80% spread, every other turn I will hit you with all four launchers for about 40 damage, while you will miss at least once with the third LRM launcher and you will average ~45 missiles hit when all three hit, while only averaging 30 or so if one launcher misses.
ATMs aren't really a niche so much as they are an alternative way of doing direct-fire damage. And BattleTech has quite enough weapons that do direct-fire damage. Preeeeetty boring as a FASA Field Manual expansion weapon, if you ask me.
You're welcome to that opinion, but it does nothing to negate the fact that ATMs can be quite effective.