Ah I see your point. All I know about haikus is from this source.
After highlighting the most important characteristics of a haiku it states:
[...] form is not nearly as important as the other strategies I've covered. Form, in fact, is the most misunderstood aspect of haiku.
Since most english haikus I come across do not adhere to the strict format I assume it's true.
Initially I stuck to the pure 5/7/5 format and that was fun, but after some experimentation I appreciated the greater flexibility which allows for a better rhythm.