Simply unnecessary wear with a good chance of potential damage to the tire due to stretching each time you swap the tires off the rims. Just a personal thing I wouldn’t do to my tires (for this reason). Factor in the cost for the mounting plus balancing each time, doing this twice a year adds up. I’d rather spend the money on a dedicated set of wheels for winter vs risking the pretty summer set.
Another big factor too in ones decision is space. I’m lucky to actually have space I cleared out of my garage for a set of wheels. Other people simply have too many things (lawn mowers, kids toys etc
) or no garage at all. In that case, get the best all seasons you can get and be done.