Well you're not the only one
This topic tends to come up alot XD And it basicly always ends with "It is your item (outfit in this case) and you are of course free to do whatever you want with it".
Personaly I love MOC/MIB G1 ponies and outfits as the artwork on the boxes are stunning and Id never want to destroy that by ripping into the package.
I also believe that when it comes to G1s the MOCs/MIBs are few and far between and as there are people who enjoy them like that Id just buy the pony/outfit loose if I rather want it like that to save one of few MOCs/MIBs that are left of the G1s.
G2s and G3s seem to have a much higher number of MOCs/MIBs floating around. Especially the G3s so Im not personally all that concerned about them. Plus the artwork isnt as pretty
Then again, they're no longer in production either anymore and the numbers and chance of finding them like that will go down if one opens them too.
As for G4s that are still in production - go nutts XD I keep some of them MIB as I prefer them that way, but thats the only reason.
Those are my thoughts on the subject but at the end of the day, as so many have said before me... It is yours to do what you want with