I am a PADI DM and I know PADI does not have a full face mask course, but I dive using a full face mask because I feel it is more comfortable and possably safer than a standard regulator and SCUBA mask. If you know how to use something, or you get training on it without going to a LDS or national agency instructor, then by all means use the tools you know. PADI just offeres classes on equipment and techniques that are not main stream SCUBA so that anyone can learn about them. Knowledge just make for safer divers reguardless of you certification level.