It is a weird psychological thing for me, iv'e recently bought a MOTU Battle Cat, found a nice box and successfully sealed it back up, it all looks pretty much new just the same as a MISB example, it looks the same and displays the same but cost me 1/4 of what it would have been if it was MISB. I love looking at what iv'e resealed and it gives me as much enjoyment as it would originally sealed, but the difference is deep down I KNOW the toy has been out of the box, I KNOW it's been handled and probably played with over the years, I have this thought in the back of my head whenever I look at it.
For me personally, paying the premium for a MISB means I get the personal enjoyment of knowing the toy has never been touched, and knowing that the box ( or MOC ) has never been opened since the last time the toy went in there. There's something special about knowing that it's been fully preserved since that day and that it's been passed down through a few different collectors who all took care to look after and preserve the seal which could easily have been opened by anybody in an instant, but instead chose to preserve it's original state, and the fact that not many of these toys have survived this journey and the guaranteed eventual plastic/glue degradation is what makes it so special. That packaged plastic thing that you ripped apart once 30/40 years ago and loved more than anything else, there's still one out there somewhere that never got to be opened and now you own it again, still as it was when your mom/dad picked it off a shop shelf.
All of this is why I pay the extra cost of an original sealed toy, instead of just paying because it's the original 'glue'.