Knocking out the side torso section's internal structure also results in engine destruction, with an XL engine, so you don't necessarily have to get three lucky criticals (though you can do that pretty quickly with LBX ACs, MGs, and missiles, as they either score multiple hits at once or fire very rapidly). You can focus on taking out any
of the torso sections, rather than being limited to blowing out the center torso or head for a kill.
You've also got to remember that in the MechWarrior video games, this is a bigger deal, since pilots can deliberately aim for certain sections of a mech. It's not like the tabletop game, where you've got to make your hit roll, and then roll on a table to see which part of the mech you hit, and so on. It's reticle-over-target-pull-trigger (plus or minus some lead angle for non-laser weapons). If I suspect you're toting around an XL engine, then I'm going to pile fire onto the weakest side torso section, and it's only going to take a couple volleys to end the encounter. If you've got a standard engine, by contrast, then shots to your side torso sections don't necessarily do a lot to bringing me closer to victory.
Even setting aside total engine destruction, though, a single critical hit to the engine has pretty nasty consequences. You're slowed, albeit slightly, and build up gobs of heat whenever you make make any kind of movement. With an XL engine, you've nearly tripled the likelihood of taking such a hit, versus a standard engine. In the most contentious, knock-down-drag-out fights, that kind of vulnerability puts you at a huge disadvantage. In a fight that should otherwise be an easy victory, a lucky through-armor critical hit on any part of your torso
can quickly even the odds for your opponent.
I stand by my position that the vulnerability of an XL engine isn't worth the tonnage it saves you. Mounting a standard engine is like having that many more tons of armor keeping you alive.
Those ads all over the main menu or whatever it is. "BUY TEH FOUNDER PACK KTHX" :|
It's a free-to-play game. Of course they're going to advertise what you can buy in the game. Piranha needs people to know what they can buy, so that people can buy it, and Piranha can pay the bills to keep the game going. As long as it doesn't impede your ability to actually play the game, then is it really that off-putting?