I've busted open 4 or 5 of them for cards I needed for my Herschel Walker collection. The main issue with slabs in my case is storage space. I currently have over 850 different Walker cards and keep them in regular cardboard storage boxes. If all those cards were slabbed, I would need several times the storage space.
I do own a few slabbed cards. I have a Joe Montana and Walter Payton rookie, among a few others. I can maybe see it from a card protectioin standpoint, but I've never liked the whole "grading" thing. As many cards pass over a grader's view in a day, it's hard for me to believe that a subjective grade can mean many thousands of dollars difference in certain cases. The chase for as many different cards as I can get is the fun for me. Not going to cry over a slightly dinged corner.
There's no intrinsic value in cardboard, but there's a lot of value in doing something you enjoy!