I refuse to use mortars for the same reasons. I think most players are on the same page. Using mortars is seemingly a mark of shame. Using them either indicates that you are completely unskilled in the game, and have to rely on a weapon that can clear multiple mechs with a single shot, or that you are more concerned with climbing ranks than the game being enjoyable, so much so that you're willing to play in manner where you ignore anything that actually creates a challenge.
I get that the damage by an EM rifle can be frustrating, but I'd say it's still easier to avoid than getting hit by rocket mortars, and there's multiple ways to avoid it, or to get to and take out a mech with them, where a mech with rocket mortars can typically clear the field repeatedly before any mech has a chance to get to them, especially if they have any backup at all.
I just don't see where the argument that EM rifles are overpowered and unbalance the games comes from. There are so many ways to avoid them or take them out, including using EM rifles, which a lot of people choose to use as a means to balance it out. If everyone who purchased EM rifles and upgrades for them instead chose to put their money into other weapons, like disc launchers, javelins, or in particular, rocket mortars, I think the results would be even worse in terms of how unplayable matches would be. EM rifles just create a particular challenge because of their popularity, especially once the developers gave them to bots.
From my experience, I just don't see the unbalance from EM rifle. I use 16's and 12's on two different mechs, and a lot of the time, I'm able to get the most kills in the match, or at the very least on my team, but my win percentage is still only around 60-65, a pretty even amount. Having them can help a player score a good amount of kills, especially if you know how to play from long range, but it's not like having them, even multiple mechs with them, is a guarantee to win. They're a specialist range weapon, if you're dealing with close quarters, maps with a lot of cover, or multiple mechs rushing you, you might take down one, maybe two, by firing and displacing, but there's still a good chance you're ultimately going to lose out, especially against someone who knows what they're doing with a panther, killshot, surge, javelins, or disc launchers.
Maybe in lower rank matches, they might be an issue, I'm not sure because I didn't have them when I was playing at those levels, but at higher ranks, most people and bots seem to have good enough mechs, weapons, and skills to offset any great advantage that they might bring.