It's one of the rules for speedmapping events, but if i think about it, that is also probably the rule we break the most in speedmapping, so i can see why you put it out and say that it is optional, apart from the fact that the theme on speedmaps by tradition was always optional, we just got accustomed to follow it.
Looking at it from a different perspective it can lead to an hordish combat map or a big with almost no detail map, and make that the theme. Hmmm
... maybe i'll do another one, now that i have a theme the ideas pop in my head way faster.
And lastly, ..., i am sorry for you, but i am not the Coco, i ain't going to eat you no matter how much you call me like that :)