I think as others have said it depends on your hair type - I have type 4b(c) hair and although when it's natural it can be made to look bone straight (esp if you go to the salon and they use the super hot tongs), i think in reality it takes a lot of heat and time, which for me, was far too much effort to be doing it on a regular basis. It also reverts fairly quickly - forget humidity, sweat - exercise, rain - going anywhere without an umbrella.
Meanwhile as a relaxed-head rocking a braid-out it is a lot less work than straightening natural type 4b hair and would lasts a lot longer. The braid-outs don't look like my type 4b hair but more like a type 3b afro.
I actually think those with type 3 hair can flick back from straight to curly with a lot less time and effort than a type 4 head person - (assuming hair length was the same for both)
So perhaps the spectrum is wider for naturals who are type 4, however due to the time it takes to go from one style to the other, I was less less likely to go from curly to straight when natural. With my relaxed hair I go from straight to curly much more often by setting my hair on my satin rollers.