As always, it all comes down to the practitioner, not the art (or stance or guard). In MMA people have fought with all kinds of "unorthodox" stances and low guards (Machida, Wonderboy etc.) and have had success using them.
As @fluffball mentioned above, the way you move your whole body (footwork, dodging, leaning, pivots etc.) is way more important than your guard.
Also remember that stances and guards are fluid concepts, they change every time you move or strike so don't get overly attached to them.
Many people (including myself) draw comparisons between fighting and chess. Note that in chess you don't strive to build and maintain a certain pawn structure for the entire game - instead you learn general principles and tactics of correct placement of pawns and pieces and constantly react to what the opponent is doing. Fighting is very similar.
Stance and guard are distinct. Stances are fluid, guards tend to be more fixed, until the later rounds.