[Don P]

We have a slide release on all modern semi-autos that I can think of, yet some people chose to release the slide with their other hand.

Is there a reason for this beyond personal preference

Actually they are slide LOCKS. Yes you can release the slide depressing them and IMO that puts them under greater wear.
We were instructed during training with many LEO's and LE instructors that, under stress motor skills diminish and trying to hit the slide lock to release the slide may be missed, and here is the point they made- you miss the slide lock lever, you take your eyes off the target/person and look down at the gun in order to release the slide, you now have regain site picture/focus and the target/person being engaged. If you sling shot the slide you maintain eye contact with the target/person. Its almost improbable that one would miss grabbing the slide to sling shot it closed. I tend to agree with them using the sling shot method during competition matches, sling shot works.
As a side note I will RO shooters during a stage and watch them go to slide lock, change mags and try to hit the slide lock and miss it then grabbing the slide to sling shot it.