This has been posted many times.... its also common sense that you can do this with most professions..
For example, lets say you really like the look of "deepdive helmet"
Deepdive Helmet - Item - World of Warcraft
So you level your engineering, and wear your helmet. You decide you don't want to be an engineer anymore... so *while wearing* your helmet you go change your profession. You still get to wear the helmet with the +15 stam, but *do not* get the engineering buff of unlimited water breathing... However, if you ever take the helmet off, you can no longer put it back on since actually putting it on requires engineering.
common sense.