Sure, the polarizer will protect just as well (if not better - there are two layers of glass in a CPL). For those who do stack, it's either laziness (not that I'm accusing anyone here of that!), or simple lack of time (needing to get the CPL or ND on quickly to avoid missing a shot in the time it would take to remove the UV filter). In some cases, it might be "the fear of the 30 seconds the lens will be unprotected" - if I was shooting on a beach with crashing breakers and lots of salt spray, and wanted a CPL to cut the glare or an ND to blur the water with a long exposure, I would just screw on the second filter, rather than removing he UV filter and needing to clean off the front element later. Then, if I needed to keepshooting with that lens once I left the beach, before I got a chance to properly clean the UV filter, I could just remove it and keep shooting.