My eldest daughter swims a lot. One of the pools she swims in twice a week is in a school and itt doesn't have many cubicles - about 12 I'd say.
There is always 1 group coming out, and 1 group going in - so double the amount of people in the changing rooms.
Children leaving their things in the cubicles has always been a problem, despite the signs etc...but that's not really the problem.
The comparison with the café tables is as follows...
Often the parents of the children leaving the pool often go to the changing rooms as the lesson finishes, empty the child's locker into a cubicle. The child then goes and has a shower. Now, there are only 3 showers, lots of children, so what can happen is that the parent is standing outside the cubicle, with the child's things in, for 10 minutes which child waits on shower, showers, chats etc...
So there are children running late for the next lesson who cannot get into a cubicle to get changed, and children who don't shower at the pool needing to get changed (my daughter always showers at home), and the children who have been first in the showers and are finished waiting to get changed, whilst parents are reserving the cubicles for when their child returns from the shower. (Sometimes the parents accompany the children to the shower to help with hair etc so leave the cubicles filled but unattended)...
So - what is right and what is wrong - back to the café table scenario - the parents have ensured that their child has a cubicle to change in when they are ready to change, however as the child isn't actually ready to change, they are preventing others who are ready to change from using the cubicles...
(these are popular swimming lessons - approx. 24 children in each session - I don't know what happens in the boy's changing rooms - I can only speak for the girls...)..
So, simply, cubicle reservation or no cubicle reservation....