I wouldn't go out of my way to buy a MOC/MIP pony of a generation that is not active at the moment. For one thing, they're much more expensive and cost more to ship, and I want to feel like if I can play with my ponies whenever I want to. I can't tell you how many times I've groaned over finding G3 Winter ponies for sale, only to find out they're MIP.
That, and I LOVE seeing how packaging and brand images evolve over the years, and MIP ponies are an awesome way to do that. If I got a MIP double of a pony I already have, I'd definitely leave them inside and just bask in the wonder of feeling like a little girl in the 80s (or late 90s, or 2000s) looking at the pretty pony on the store shelf.
HOWEVER, I actually went for the third option. The truth is, more than once I've come across MOC ponies that are just BEGGING to be taken out. Either the backcard looks nasty, or has a ton of rips an/or tears, or the plastic bubble has yellowed... But the most common are the ones that you can just tell the plastic hasn't taken well to years and years of being sealed up.
For example, G2 Hip Holly and Copper Glow are infamous for their uncanny propensity for stickiness. Now, no matter how much I want one (and it is a LOT), I am still afraid of ordering a Copper Glow because I don't know if she'd react well to being confined in an envelope for the 1-2 weeks it'd take her to get to me - I can't begin to imagine how one that has been sealed for over 10 years is doing.