But only as often as the man wears a kilt.
Any reason for en somero instead of dum somero?
The difference is the en often refers to an event which occurs or repeats within a time frame, while dum usually refers to something which happens for the duration of that time frame.
Shouldn't it be "in the summer"?
Why is it that we don't need to use la somero, but just somero?
Because we're talking about summer in general and not about a specific summer.
Shouldn't the common word order be "En somero ofte portas la virino jupon."?
The common word order is subject, verb, object, and since la virino is the subject, portas the verb and jupon the object, the given word order is the usual.
The word order doesn't matter in Esperanto anyway.
Word order is freer than many other languages, but there are definite limits.
This is another sentence that could sound more like English if we just changed it to "that woman" or "women"; otherwise it sounds bizarre.