The problem (thanks to Geo's card analogy) is that, unlike a card, a slab can't show you the overwhelming majority of your property. Unless you really, really love the cover, you basically own a box with a picture of a (presumably) famous comic on it.
Plus...I haven't seen a slab up close, so I'm going out on a limb here, but I'd guess that they're not exactly "archive quality." For a sixty-year old comic to continue to be in mint condition, you basically need to keep it from oxygen, moisture, anything that might contain any acidic traces, and...oh, a whole bunch of things your local librarian will be happy to lecture you on if you ask sincerely.
They do claim it has been "tested for archival quality," but I don't actually know what that means (the engineer in me points out that testing doesn't suggest success, and a grading company should know enough to tell us who conducted the tests), and doesn't sound like something that costs in the neighborhood of a hundred bucks or weigh three quarters of a pound. Their FAQ points out that they're easy to open, too, which definitely doesn't suggest much security.
Of course, that's not their business, so I wouldn't expect it. Their business is in securing transactions: If you want to buy a book and can't inspect it, you have presumably-impartial third-party eyes on it. And the slab itself guarantees that it's the same comic that was inspected...well, except that the slabs are easy for greasy fingers to pop open and re-close (again, according to the FAQ), which...I don't know where that puts the value.
If you're not trying to make that kind of sale, though, I don't see the value, and there are far more effective ways of preserving the paper that, again, your local librarian can probably either explain or point you to a professional archivist who can.