For Battlemechs ... I think the hard points tend to determine the usage as symmetric or asymmetric. For example, some mechs have limited hard points on one side ... or have hard points separated by type. Ballistic in an arm and several energy on the other side of the mech. If you don't use the ballistic hard point then you might as well use an asymmetric build.
The other aspect is boating. Folks often prefer boating either the same weapon or the weapon type (B,M,E). Many mechs have these weapons concentrated on one side or the other ... so by choosing a specific weapon you get an asymmetric build.
Finally, you can use one side as a shield if you are decent at torso twisting. Folks chew through the arm and the side torso while you preserve your firepower on the opposite side. Of course this runs a risk that someone fires at you from an unexpected direction and can take out most of your weapons. On the other side ... you tend to be able to preserve your firepower later into the game in some cases.
However, most folks aim for the torso, so to take advantage of shield effects you need longer cooldown weapons and you need to practice fast twisting in a chassis that has significant arms (i.e. it doesn't work well for non-humanoid mechs like Stalker or Marauder).
Edit: As for battletech lore
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... Battletech is turn based so firing arcs and facing only mildly affect what parts can be hit (can't hit front torso from behind ... and can't fire at a target outside your firing arc) while MWO is real time. The tactic of torso twisting to spread damage actually makes perfect sense in the battletech universe .. pilot aim is so BAD that you randomly hit CT, RT, LT, LA, RA, LL, RL when firing a couple of medium lasers from 200m (assuming you hit at all with both you and the target moving at running speed).
MWO simulates this in two ways ... many weapons are damage over time so that it is harder to maintain the focus on one target element while moving (PPFLD weapons are probably a design error in my opinion unless they come with some mitigating aspects like charge time, minimum range and high heat) ... target torso twisting and movement will also tend to spread damage (except for the aforementioned synchronized PPFLD weapons).
Anyway, in the Battletech universe the goal is to put together the best mech build you can to win the fight. In almost any battle your mech will take damage but the Battlemechs would not have been put in the fight unless you had to win at any cost. So ... if the best build that will help you win the engagement happens to be asymmetric and utilizes torso twisting to spread damage and concentrate damage on the empty parts of the mech ... then presumably that is what Mechwarriors in the Battletech universe would do
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... it never happens in lore or table top simply because the mechanics are not sufficiently detailed and the reasons for inaccuracy on damage application are not enumerated
Edited by Mawai, 13 October 2016 - 07:43 AM.