Buying as an investment deserves outright ridicule. The vast majority of titles won't have meteoric rises in value like Magical Chase, so any varied collection will fail to keep pace with a decent mutual fund portfolio.
Of course; hopefully the majority of collectors are not looking at this hobby as a replacement to their 401Ks or financial portfolios. I reckon it is just a fun way to invest in a hobby they like, that might also pay financial dividends in the end. If that is not the case, then yeah, they probably need to get their priorities straight...
But my main point was that if you already have an item in your possession that shoots up in value, it wouldn't really be so retarded to keep it safe, so to speak. So if its sealed, and a sealed copy can fetch you 3-4 times what a used copy would, and you know you have other ways to play, then I would consider it reasonable to keep it sealed.
For me, I like to look at the contents and play the real thing whenever possible, so I let my heart do the thinking, but I can understand why others choose to use plastic covers in their complete games, or keep them sealed if new.