It's fine. Corks do "breathe" I guess, for super long term aging. I cork all my wines, but I really think it's more for the aesthetics than needing to be done that way. My wines will all be gone within two years after bottling, so I've never had anything around long enough to actually see how it ages!
I have done small "tester" amounts of wine in beer bottles and caps, and it's absolutely fine. It's not just as romantic as opening the foil around the top, using the corkscrew and decanting the wine.