I'm not sure that it is always more reliable in defensive situations - if my gun does not reliably feed and go into battery via slidestop every time, I won't carry it. I'm sure that it takes more time to rack the slide though, and in a life or death situation where a tenth (or a hundredth or a thousandth) of a second could mean the difference between winning and losing a gunfight, I'll take the most efficient (and therefore the fastest) route. If I'm behind cover and not worried about being immediately over run, I can see racking the slide if my cover allows me the freedom of space to do so, assuming my offhand arm is up and running to allow me the physical capacity to rack the slide.
Someone with a timer should clock doing it both ways several times. Maybe even film it and post it. I'll look on Youtube to see if anyone has done something like this. The important thing is to not activate the slide stop in a separate motion and then acquire the grip and then aim and shoot, but to incorporate the activation with the acquiring of the two-handed grip and target acquisition to truly see how much faster it is.
NOTE: I am coming from a 1911 and or CZ model background and so this may not hold in reference to Glocks or clones that have a very small slide lock lever that is very close to the frame. It may be truly more reliable to rack the slide with these guns. I forget that other shooters may have other preferences than mine.

In a defensive situation, I don't think I would be concerned about the life of my slide stop at that moment.