The thread on coins and slabs surfaced some possible parallels and differences between coins and stamps. Over time many high quality and high value coins have moved into slabs, but some numismatists pull coins out of slabs after buying a grade. For most stamps slabs aren't needed, but for some stamps and some custodians they can be pretty wise.
As an example, check out this stamp image and description credit to Robert A. Siegel Auction Galleries Power Search.

This 244 condition rarity damaged after the grading lost about $3000 in value as a result of the birthed crease (SMQ for 95 $6600 stamp realized only $3700). That is a big dollar loss for an accidental paper bend. Did the stamp slip going into or out of an album mount? Who knows! Bottom line, faulty stamps are extant, and all faulty stamps use to be sound and therefore more valuable. It is the loss of preservation that drives a loss in value.
Granted it is just one example, but some could argue this stamp would clearly have been better off in a protective slab.
Those who act as historical custodians of very high quality and/or very high value stamps may choose to slab stamps as a prudent measure of preservation and financial protection. I think right now slabs are out of favor, but over the next 20-30 years a dichotomy for stamps will develop as happened with coins. Most examples of 244 being a collectible and very nice grade probably wont be slabbed. Those examples that are very high value such as never hinged and/or highly graded numerically will end up permanently housed and traded in slabs.