I've got 5 bikes that I use for commuting as well recreational riding. However, I do most of my commuting on two of my bikes that have fenders and are set up better for carrying loads and mounting lights. On fair-weather days when I don't have to carry much gear, I might commute on any of my bikes. If there is any chance of rain or I have a lot of gear to carry, I'll ride my touring or sport touring bike. For recreational rides, I pick whichever bike is best suited for the route, which might be my racing, cyclocross, touring or sport touring bike. I like having various bikes to choose for riding. On the most basic level, it's good to have a backup in case one bike has a mechanical problem or needs maintenance. It's also nice simply for variety. If buying a second bike, I would look for something that would expand your options for riding. Eg, if you already have a sport touring bike, consider a cross or touring bike.