Magic theory. Not an exact science. Yet get a few (serious) magicians together and they'll hash and rehash the reasons behind the success or failure of various ventures, deconstruct endless routines, and spout bon mots from their favorite authors.
So where does theory meet practice? Do I discard theory altogether? Is Henning Nelms hopelessly out of date? Does Tommy Wonder's advice only work for him?
It comes down to this: magic is applied psychology. And psychology is neither a hard science nor an exact science. It's constantly being refined as we better understand ourselves and as the dynamics of human interactions change.
So it means that magic theory, like any theory, must be tested. If we really want to get our ducks in a row we ought to change our language. Things fairly well tested are theories. Relativity. Evolution (oops, going to catch some flak for that...). Spectators tend to focus on things that move rather than things that sit still, in the absence of other stimuli.
Those things not yet satisfactorily borne out by practice are postulates, not yet theories. Red cards are better than blue (or vice versa). One should never (or always) write one's own magic script. You shouldn't do card tricks for kids.
Why worry about magic theory at all? Because it can save a lot of time - otherwise each routine, each move, each line has to be developed in vacuo, with nothing to inform as to it's value other than empirical testing. That would be fun, wouldn't it - having to do market research on every facet of a routine before it became audience-ready? Also, understanding your personal theoretical underpinnings will help you develop as a magician. You'll know your foundation and will have that foundation to build against.