Yes. In the slab, only ~ 5% of the book is visible. There are a whole other 34 pages just brimming with artwork in there (usually more if we're talking Golden Age) that you can't see when the book is in the slab. I want to have access to 100% of a book I own. Some people don't care. Those that do become slab-crackers!
Also, I like to change the microchamber paper occasionally, which requires deslabbing. Also, I really enjoy the 'conservationist' feeling I get from putting 3 or 5 sheets of microchamber in there, rather than just 2.
More generally, there are arguments to be made that storage in mylar in an acid-free environment is "better" for the longevity of book than sitting in the slab, but opinions differ. As an aside, one thing that is clear is that books can be damaged in various ways by the inner well of the slab...putting them in mylar takes that out of the equation.