It's not really a valid question, is the answer. The devs and anyone else involved with creating new designs and configurations are the reasons that the designs are what they are. Its all fluff, nothing more.
A clan lrm20 is 5 tons, 6 heat, with an average roll of 7 you are hitting with 12 missiles per launcher. Each ton invested is going to net you over twice as many points in damage on average. An er ppc does the same, but generates over twice the heat. A Gauss rifle is only going to net you 25% more damage then the tonnage sunk into the weapon, but with 5 less heat, but has a min range.
Some point to the cluster roll as a reason why. Some are even adding in the concentrated damage of an er ppc or gauss rifle, while suggesting that the single location struck is a good reason to use them over lrms. That thinking is flawed outside of the risk of a headshot. Its flawed because all of the damage is hitting a single location, so if you miss the location that you wanted to hit it means nothing. Hitting a fully armored leg of an assault mech means nothing when you needed to hit the CT for a few points. Since the most likely roll is a 7 the clustered nature of lrms means that you're likely to hit that ct with those clusters. It also means that you avoid the "all or nothing" nature of a 15 point slug.
In all of the games that I played last year, an average of 2 games per month, there has been at least one unit killed by a headshot. Most of those have been gauss rifle slugs to the head, but there have been some that have been hit a few times with smaller weapons, and one srm that killed a pilot by rolling a 2, then 12, then an 8, and I believe its 5 for the pilot on a fresh shadowcat. Do headshots happen? Yup! There is that question if the headcapper is worth it since it does happen, but then again the most clusters you have to roll, the more spins you get to win.
If I am running clan units, the deciding factor of what variants comes down to BV and my view of the unit. I know that clan er ppcs are great and their LPL is OP, except when I use it since that unit is always out of position somehow. I mess with I.S. LRMs and indirect fire so using LRMs without that stupid min range for just a handful of extra BV is an easy weapon choice. I can fire 3 lrm 20s and run with my standard 10 dhs, or i can fire a single er ppc and run before considering how much heat I'm going to build up when i fire another weapon. For 3 tons over the weight of a gauss rifle i can use 3 lrm 20s and average over twice the damage of the GR with the potential of doing up to 4 times the damage. Now a GR and ERPPC combo would be close to the tonnage, heat, and average damage of the 3 lrm 20s. At that point its a matter of dispersion and potential additional damage. With my personal experience and preference, my choice would be the lrm launchers as there is less of a chance of hitting all of the wrong locations and I gain indirect fire on top of it.
One last thing, the clanners don't fight as they say on the box. Either they have no idea what zel is, or they say that they will but declare that they are playing as x clan who doesn't adhere to it unless its to protect themselves. Also, fluff wise, they abandoned it. That means that clanners are not "above" using indirect fire. Being able to hit someone when they cannot hit you is a big bonus. You can have all the 15 slug weapons in the world, but if I can melt your unit with indirect fire before its able to establish LOS against my lrm unit, it means nothing. Post Zel death the clans really should have put more lrms on their units. There should be upgraded LRM carriers, but there isn't any. Why? Because they don't make sense, or because....Devs say so? :drool:
