Why do I detest fixed width? Two reasons...
One - I feel completely hemmed in and claustrophobic having to use fixed width skins, I refuse to do it in fact, for example: There is an excellent Admin forum around, even though it is a fine site with good people and discussions.. I cannot abide fixed width skins, truly physically feel sick using them, and whilst that site several choices in themes, they are all fixed width, sadly I cannot post there at all. Some people have said to me that they cannot read great big long strings of text, well I cannot read great big horizontal blocks of text.... different mindsets, left/right brain differences.. I'm a retard, who knows, but I do know I am not alone, with 8 years of providing skins to boards of all levels I have only twice done Fixed width Custom Skins, and I detested every minute of doing them. I have had this discussion with many many people over the years, and lots feel the same way.
.... And Two... it is often control freakiness of the highest order.. dictating what width people will read forums is imho the Admin Nazi-est move out there... if I want 1920px wide text, surely that is my business.... and the argument you don't want to "maintain" two styles is (naturally) spurious... make the default xenforo style available (which is nicely fluid you will notice) then you don't have to do a single thing.. And then, those of your pooor members who like me detest being dictated to in such a fundamental way will have one less control freak thing to object to
It costs you nothing, it delivers choice, and everyone is surely a winner.