i'd hold onto the bike for a back-up. weather not withstanding, it's always good in case one bike is down for repairs.

in my own case, i use one bike all year for every weather and ride. it's a joy to ride, and so i do. winter? change tyres to winter tyres if necessary (Nokians, 10 minutes); keep drivetrain clean and lubed; make sure fenders are OK. summer time? sometimes i take the fenders off, put on the sportier 28mm tyres. i could ride one of my back-up bikes, but they're not as fun or dialed-in. better to make my ride as perfect as possible.

when the weather and time finally kill the frame, it'll be time to get a new one (i.e., years from now). i can live with that.