Depends on the item. Protection-wise I certainly can appreciate slabbing some older photos. Heck, I have an 11 x 14 Dana original of Jim Jeffries that I'd love to have slabbed for protection. From an authentication perspective, I find having the cert as the capsule around the item more convenient than having a separate paper cert--can't tell you the number of times I've fumbled around my storage area looking for that damned slip of paper to go with the item. That said, the slabs can be bulky and inconvenient. I guess what I'm saying is that I would not buy or refuse to buy an item just because of a slab.
On a total tangent, I liked the capsules that CGC was experimenting with for a while with photos--they were about as thick as a semi-rigid but fully sealed and held the photos safer than an album page. They had been doing it with lobby cards in 2011 as well but they might have dropped that. Too bad, seemed like a decent lightweight alternative.
You know what might be great too is a large format semi-rigid, sort of a card saver for larger format items. I don't like the top loaders because items slide around but I use them as a default.