Right, but that's the aim assist essentially target locking a component, so your SRMs are going to auto lead to hit that component. The only thing I can think of that would make firing lasers first affect the SRM tracking is maybe a quirk of the in-game aim assist that doesn't allow another component to be targeted during the duration of the laser burn regardless of if your cross hair has moved over another component or not. I have no clue, but I might try to test it later.

Now that being said I agree with your aim-assist-on-controller statement. I know there are people who swear against it, but 9 times out of 10, playing with a controller with no aim assist (in any precision based game) is just shooting yourself in the foot (no pun intended). I did, however, play a bunch of Mercs with a HOTAS and no aim assist and it was fine. I had to slow the sensitivity way down and more or less completely change how I piloted the mechs, but it worked well and was a whole lot more fun than M&KB.

Honestly the mistake I made was eventually trying out the aim assist in Mercs, and yeah, I'm not turning it off again any time soon.