I don't know about that mom, but I can speak about my own experience.

For the last three years, I've ridden just about every day while pulling a trailer. I did this because I bike commuted my kids to preschool/kindergarten (I'm not commuting right now due to having a newborn on my hands). Since my husband works so that I can stay at home with the kids, I'm the person responsible for getting them to and fro during the day. I am WAY stronger than my dh when it comes to towing! There is absolutely no comparison in our ability levels. I have far more stamina, speed, and 'wind' than he does--and that is due to all the biking I do. He's probably put 500 miles on his bike in the time that I've put 3,000 on mine.

When we do family rides, I tow the trailer, he tows the trailer-cycle. He gets the heavier child, but of course, they can actually help power the vehicle, so it's not as bad as it seems. Even so, I find it can be a bit challenging to ride with a trailer-cycle due to the awkward lurches that happen when the child leans or turns, so I'm happy to have him do the lion's share on that particular accoutrement.

I think that many times the mother is the tower due to the fact that she is usually the person who is responsible for caring for the children. She's in the habit of communicating and caring for the kids, so she automatically takes on that role when it comes to biking. Also, if a mom isn't working, she's the one with more time to bike--and needs to ferry the kids--so again, she is the tower by default. Many trailers have special attachment brackets that don't allow you to easily/quickly change the trailer to another bike. If you have to choose one, you'd probably choose the bike that gets used more to travel with kids, right?

All that said, I think it's a shame when a family splits up due to ability levels. The stronger rider should absolutely share the towing duties (buy another bracket or upgrade to a better trailer that has a universal hitch!) AND should pace with the rest of the family. The only exception to this that I can foresee is when there are several biking members of a family riding together. Sometimes it's nice for the older kids to forage ahead of the smaller kids/parent(s) to avoid boredom due to a pokey pace. I must admit that there are times I get frustrated over the speed (or lack thereof) of my 6 year old son!

Jane