Depends on the collector buying it. If you're not looking to sell it any time soon, and it becomes a pain to store separate from its mate, you shouldn't feel obligated to keep it in the slab. Depending on how long you plan to keep it, cutting it out of the slab wouldn't be the wildest thing a collector has done to maintain uniformity of presentation for their collection.
As far as value goes, there is so much fluctuation on non-blue-chip photos, I think it would be tough to predict what effect it would have on the value. Much like the photo itself, beauty is in the eye of the beholder, and some collectors will place more value on a slabbed photo than others (especially if their own collection is uniformly maintained in slabs
)