Keep it and buy another one. It`s easy to get and not that expensive. But remember: there is nothing that beats this fat 12" sound. Best sound you can get.
There is something special about the vinyl sound... but to say that "cds suck big time" is pretty exaggerated. I remember the first time I heard a new cd... I was amazed at the crystal clear sound, no hiss, etc. Now we hear the crackles of an old LP and get nostalgic, but remember, those crackles and hisses were once a menace. There is a warmth to the sound of a good record, but I must admit I definitely enjoy hearing all the highs and lows and the crispness of a well-produced modern recording... even a smartly remastered old recording. I like being able to hear what I couldn't hear before. I think the cd is the best medium technology has been able to give us. I miss LP artwork, but I think LP sound is more or less reproducible on cd. Anything lost is so minimal as to be barely recognizable.
Nevertheless, I think that Miss You 12" is a gem... Keep it in good condition if you end up opening it and listening to it. I think records are meant to be listened to... If the difference in condition means you'll miss out on a few bucks if you plan to sell it someday, then keep it wrapped, but what's the point if you're not going to sell it? I say listen and enjoy, just take good care of your treasure! Good find; congrats.
Does it have the picture sleeve? Pink vinyl? If so, I'd leave it sealed and find a copy w/out the picture sleeve on black vinyl -- it's more common and it will actually sound better if you wanna play it.
Sealed does not necessarily mean that it's new. You can re-seal a used record. I would open it and see if it's really new. But that's my personal opinion, you may prefer to leave it sealed.
Have a copy as well on pink vinyl with Far Away Eyes on the flip. Great track and sounds so much more warm and heavier on vinyl!! Great groove and brought the Stones back to the top when punks were trying to punk them...."no one can out punk Keith".