In my experience, they can fall out without it. Well, not without it completely, but malformed enough to not hold onto the pads. In my case, the pad fell out because the prong in the middle of the piston is so damaged from years of screw-driver pushing back in that the end is mangled and no long sits in the recess in the back of the pad. If this prong is fully in tact, the pad can’t fall out with the disc in (it can dislodge a bit, but not fall out).
If like on mine they’re worn to buggery, the spring clip is doing just about all the retention. I squashed mine into shape a bit, so it’s a bit of a loose fit on the caliper, but the pads clip into it with a nice ‘click’.