Plastic foam products which tightly fit the knife pose several risks: first, they can trap moisture on the knife. Second, they can out gas chemicals which can degrade or decolor knife materials and can build up as a cloudy, difficult-to-remove film on metal. Third, their material can migrate into surface pores and imperfections causing them to "stick" to the knife. Therefore, foam-lined cases and pouches are not recommended for long-term storage of valuable knives.
Cases such as the Spyderco cases that have clear vinyl have many of the same problems as foam, especially out gassing. To make what would normally be a hard plastic material soft and pliable, you have to add a plasticizer to it. The chemicals called phthalates the we hear about sometimes are plasticizers added to plastics to make them soft and pliable. We sometimes hear about them because they can come out of the plastics and go into food and beverages and drugs and such stored in or even just passing through containers made of such plasticized plastics. Some research shows that this may lose a health threat.... to people, but what about knives? Many plasticizers can build up on metal and other materials and cause a difficult-to-remove milky film. It is not unreasonable to suspect that, over time, they may affect some of the plastic materials used in some knives, possibly making them softer. Softened plastic films may also, over time and especially with even slight pressure, tend to flow into pores and surface imperfections in some materials and, thusly, tend to bond to those materials.
Cases lined with felt or other cloth have a special concern. The dyes used may tend to migrate over time into some materials. I was once shown a beautiful custom Bali-Song with genuine ivory inserts. So beautiful... until I turned it over. It had been stored for many years unmoved in a display case lined with green felt. The green dye had migrated out of the felt and into portions of the ivory. Such a shame.
Be very careful about using desiccant packs in sealed containers. Be very wary of sealed containers in general. Desicants do not make moisture go away. They just absorb and hold it much like a kitchen sponge. But, just as a sponge can only trap a certain amount of water, so also desicant packs have a limited effectiveness too. And just as a sponge will release its water if squeezed, so also desicant packs will release their water -- possibly doing serious damage to the knives they were supposed to protect -- under the right circumstances.
So, there are a few facts to consider right there.