In my opinion, having multiple bikes and letting other riders/potential cycling enthusiasts/house guest use them is one of the most enjoyable parts of being an avid cyclist. Out of my three bikes (Giant Bowery with both brakes, 1960's Schwinn Continental conversion with no hand brakes, and a 1970's Free Spirit road bike converted to coaster-brake), all except the Bowery have spent significant amounts of time being ridden by people other than myself.
After my girlfriend's brother had his bike stolen, I left the Schwinn conversion with him for a little over a month after we visited her family one weekend. After riding that bike about 10 miles a day for a few weeks straight he succumbed to the brakeless fixed-gear fever and is now scouting around for frames to use with a wheelset I'm going to be able to pass along to him.
The coaster-brake bike is mostly used by people hanging out at my house who want go for a ride on days with nice weather and don't have a bike of their own or didn't bring their bike over. Many of the people who have ridden it don't do much cycling and tell me how much they enjoy the simplicity/smoothness/quiteness of the ride when compared to what they remember riding a bike being like, which was usually casual city riding on a Wal-Mart mountain bike with under inflated tires and improper fit.