For a long time (since 2009) I have owned two bikes. One is a road bike with fenders for occasional rain. It is also equipped with dynamo powered lighting for early morning rides. The other is a dedicated commuter with racks, fenders and also has dynamo powered lighting. It has decent tires for off road recreational rides and also serves as a child hauling bike with a rack compatible with a child bike seat. In the past I also pulled a trailer with one of my boys on this bike. Do I need a third bike, which I would find useful but would also help me meet the Velominati minimum and would also help me meet the N+1 imperative and actually help my life?
This all depends on what your perspective is. Mary from Chasing Mailboxes recently talked about her stable of bikes but her situation is far different from mine, not least because her husband has a large stable of bikes so they they both are on the same page. It seems like a nice merging of interests in a relationship. While my wife rode when we met, that sort of fell apart during the years we were trying to and having children. And she hasn't been on the bike very much since that period. But she recognizes that I like to ride and we manage to make some time for riding and some room, and money, for bikes. But space in our small house is at a premium and money is tight so I have to make a strong argument for why I need another bicycle.
It's an easy argument to make at first glance. I want a bike to ride to Alewife when the weather is bad so I can take public transit the rest of the way to work. I also might consider taking it all of the way in in less than perfect weather, knowing the bike may be abused some and not perfectly maintained. (Matt Roy calls this willful neglect.) It would also be nice for short trips around town. Optimally, this bike would be a simple 3 speed with a steel frame (and I don't mean hi-tensile steel) with platform pedals and good brakes and fenders and eyelets for racks for the future. I'd carry a backpack with a couple of u-locks (because it is would be an inexpensive bike doesn't mean I want to make it easy for a thief to take it). To answer the question, "what would it do for the family?", I'd say it would give me some exercise in the middle of the worst weather.
My wife might counter with this, "you always want to add something to your bikes", which is true, especially in this case. I'd want studded tires (I know some people ride without them, but I'll chose to have the security) and they cost real money. I wouldn't add dynamo lighting, except by moving a wheel and light from another bike, so it wouldn't be excessive. Maybe I wouldn't be able to restrain myself but I think I would be able to.
A more complicated problem is this - aren't there other bikes that I might like to have? And the answer is yes. I would love to get a long tail with the children's seat that Xtracycle sells. But before we went that route, we would really need to decide if we want to be an occasional car-free family, which I don't think my wife is ready for (and she agrees with me). And more importantly, I'm not completely sure my boys would love to get around on bike. They sometimes enjoy going with me on the bike seat, to some degree. But an investment in our lifestyle like a long tail, we'd have to do it on marginal weather days and days when they really didn't want to ride. I think about our trailer, which wasn't all that big of a success. Our older boy really didn't like it. He is a talker and would be talking with me but I wouldn't hear him until he was crying inconsolably. He was fine when he fell asleep in the trailer but he would always wake up before we got home and you can guess what happened then. The second guy enjoyed the trailer but enjoys the bike seat more. And both boys are about grown out of the bike seat. We have used a trail-a-bike, which is pretty hard to ride for me but we will continue to use it. We are likely to get new bikes for the boys this year but I'm thinking training wheels for now, by their request. I'd also like to get a Ti randonneuring-ish bike one day and maybe something like the Honey All Roads bike. But that's for the future since my current bikes do just fine. At a minimum, for much of the riding I do they are fabulous. So there isn't any new bikes that I want to have now and commit space and money to.
So we made a decision and an inexpensive 3 speed with decent brakes and a decent frame is on its way. This mythical 3 speed I am thinking about might be fun to ride with the boys and maybe it will help me to motivate the boys to ride. I'll share more about this bike when I've had it for a few weeks or a few months.