Well like Nygma said, with smaller figures on a card, it makes sense to keep them MOC, especially if you have doubles.
But when it comes to high-end Sideshow and Hot Toys figures, opening them does not diminish their value. In fact if you have all the parts and packaging you can still sell the figure as 'mint in the box.' That's why these companies make 'collector friendly' packaging, meaning that other than maybe one tab, nothing about the packaging is damaged when the figure is removed.
I see no practical reason to keep 1/6 figures unopened forever.
But when it comes to high-end Sideshow and Hot Toys figures, opening them does not diminish their value. In fact if you have all the parts and packaging you can still sell the figure as 'mint in the box.' That's why these companies make 'collector friendly' packaging, meaning that other than maybe one tab, nothing about the packaging is damaged when the figure is removed.
I see no practical reason to keep 1/6 figures unopened forever.