My stable grew because I started with a 35# mtb and wanted something a little quicker and lighter and from that something even quicker and lighter. Each bike still serves a purpose and each is still ridden for various reasons/terrain. The road bike I got to get faster on the mtb. The mtb tandem came about because my son was too big for the trailer but not big enough to ride his own for more than 7 miles and certainly not on the road (he's 5).
Occasionally I get a comment about how it's too dangerous to have a child stoker but nobody says what age is acceptable to ride the back of the tandem. He's gone 2000 miles on both road and dirt and loves to ride.