I can think of two good reasons for keeping decks in plastic zip-sealing bags.
1) Protection from water damage, as mentioned in the flood story above, or damage from contact with other harmful agents like dust, dirt, fingers, etc.
2) Protection from paper-eating insects. When I used to go camping every summer, bugs were a major issue to deal with.
For both of these cases, if they present problems for you, it's very practical to consider buying decent-sized air- and water-tight storage, such as the ones used for food storage, or even better, camera equipment. You don't want to go too large, though, because the larger the seal area, the greater the chance of a failure of the airtight seal. Watertight camera equipment cases are also usually insulated inside with foam padding - cameras, lenses and related gear have the same environmental storage requirements as playing cards in many ways. A really good one is also highly crush-resistant.
The main drawback to these solutions, though, is that your cards aren't on display - they're in storage.