I've noticed alot of all purpose flour used in french bread doughs. Does anyone prefer all purpose for this dough, and if so, why?
AP is an 'all purpose' name which can refer to flours that vary considerably in their protein content and suitability for french bread doughs - particularly for baguettes.
One you can trust is King Arthur's All Purpose flour. There are others as well, but I can speak confidently about this one.
Why? You are looking for a flour that will produce classic characteristics of french bread: thin, crisp crust, and light airy interior crumb.
Flours with a low protein content - below 10% - are weak in gluten and thus unable to support a big, open-celled crumb. They are, however, excellent for cake doughs. Flours with protein content above 12% are too strong in gluten, and so also unable to produce a big, open-celled crumb. But they are preferred for tight-crumbed doughs such as bagels and pizza.
So, for french bread-type doughs, a flour with a protein content between 10 -12% is preferable. And that generally - though not always - is what is referred to as an 'all purpose' flour.
So AP flour will achieve a larger hole structure than bread flour?
(As noted, the names "All Purpose Flour" and "Bread Flour" are marketing terms that mean something slightly different for every brand; they're not standardized terms.)
In any case, don't think you need to use "Bread Flour" for everything just because it's got the word "bread" in it. (Some people like this immediately: "you mean I can legitimately thumb my nose at the marketing folks?" -- while others take a little more coaxing.)
What matters more is "gluten/protein" content, and you'll want to use different flours for different kinds of bread. (My personal quest -not necessarily recommended- is to use AP flour for all my "Artisan" bread baking ...after all, the original "Artisans" didn't even have high-gluten flour available to them.)
"Bread Flour" is great for pizza dough, bagels, etc. But for many other things it's just a "crutch" ...and often a rather poor crutch at that. It makes it awfully easy to bake bread that's "too chewy".
Real French bakers use an entirely different range of flours (ever hear of "T55"?) that mostly aren't easily available in the U.S. Ever since Julia Child opined that "All Purpose Flour" was a reasonable and fairly good substitute, it's been recommended for making baguettes in this country.
Even with AP flour you can make a "brick" ...or something less hard but with no holes. In other words using AP flour doesn't guarantee a large hole structure, it just makes it possible when you get other things right too.