There is a prevalent idea within some martial art (Aikido for one) that Hakama were worn low enough to hide the footwork of a samurai involved in combat. After all, if the feet are hidden, it is harder for an opponent to predict where the attack will come from. For example, see this answer on MA.se about Hakama….
I strongly suspect that "samurai wore long enough hakama to hide their feet" is a modern urban legend. It does not sit well with the Japanese cleanlinesses: They remove their footgear and wash their feet before heading indoors yet have hakama that trail the ground picking dust and dirt…
I am looking for historical primary source, the more martial the better, about the accepted length of a hakama for a samurai.
Clearly, this might have varied between time periods so I am tagging this as sengoku and meiji restoration to narrow the time frame.