Having a barge, I pay little attention to the narrowboat world. However, when I hear that my name is being taken in vain...
Of course gates should be left open. The advantages of doing so are clear.
* It saves effort. If you close the gates, and the next boat through is going the other way, its crew will have to open them again. Two avoidable activities. If the next boat is following you, yes its crew will have to close the far gates, but then it will have left the gates at the previous lock open so its crew will do no more work in total.
* It saves time. On half the occasions when you reach a lock, you will be able to go right in, saving maybe half the locking time by not having to stop and open the gates.
* It's safer. Crew can board while the boat is stationary in the lock, rather than leap on after closing when it has exited and is moving.
* It's traditional. Working boaters always left gates open, for the reasons above.
* It forces the navigation authority to do the maintenance. If the authority can't inconvenience boaters by enforced gate shutting, then it will have to fix the leaks.
The alleged disadvantage of leaving gates open, waste of water, doesn't stand up to examination. Locks are designed to be left with gates open. If a lock is maintained properly, its gates, chamber, and paddles will be tight. Of course, if you encounter a lock with a serious leak (or a notice from the authority), you should close the gates there—and report it. But nowhere else.
Leaving gates open isn't laziness or selfishness. It's common sense, and benefits the whole boating community.
I did indeed spell all this out in detail in a Waterways World article a few years back. And a few months ago, veteran boater and waterways author John Liley made much the same points in the same publication. Naturally.
PS Working boaters left the paddles open too. That's also sensible. It forces every arriving boater to make sure all the paddles at the far end are closed before running any water, and so eliminates the too-common situation of paddles being open at both ends. And, after all, crew have to walk to the far end anyway. To close the gates.