Yes, I have. Three, technically four, I guess.
When I first got back into collecting as an adult in 2003-2004, I bought a MOC Sugarberry from Darkhorse on eBay for $56 (I thought that was a lot for a MOC at the time) and opened her the minute she arrived. I remembered having her in my hands at a store when I was a kid and being told I couldn't have her, so it was just a thing I needed to do. I also bought a MIP Kellogg's Baby, and my husband's (then-boyfriend) mother bought me the Chuck E. Cheese Baby MIP. I opened them both. (Oh, the days of keeping eBay open in a **SEPARATE WINDOW** in Internet Explorer on my PC and watching the 2,000 My Little Pony auctions as they ended. Impossible to do now with 20,000 results.)
Would I do that now? No. The desire for new Sugarberry would have passed eventually, and I'd have traded the MIPs for mint, opened ponies. I do have plans to open the remainder of my MIB G3 collection. I just need space and a good method of display.
And, as for the fourth... I won a "MOC" Greek Skydancer at the 2008 Pony Fair. She was partially opened when I won her. I entered the raffle on a whim, not expecting to win. A couple of moves later, her bubble glue was so dried out that the bubble was almost completely detached and was tearing the card paper where it was still attached. I made the decision to just take her out to try to preserve the card as it was.