A very correct observation, though you're not seeing the entire picture.
First of all, openings do not win games at the club levels ... they merely get you into playable middlegames with equality at worst and a slight advantage at best. They are not "silver bullets" for winning chess, contrary to what most people would like to believe :)
Continuing along a line of theory/book moves usually implies best (what is considered best for a given moment in history) play on both sides. Note that theory in this case does not mean just an opening book but any line that the databases reveal as "still being played by Masters" with atleast equality if not some positive results. In other words, just because the MCO/NCO in your hand doesn't contain it, it doesn't exclude it from theory.
Adding to that, theory is not just move memorization, it is knowing what kinds of pawn structures occur in this opening, where the pieces are optimally placed for both sides, what the attacking plans are and what kinds of endgames occur.
Therefore, one would treat theory as gospel as it is the result of a churn of over 2 centuries worth of Master-level games, analysis and more recently, computer verification.
So when a deviation from theory occurs (in this case, by your opponent), I see 4 logical possibilities:
a) He has played a novelty that Masters have not played/published yet AND this novelty move is NOT A MISTAKE/INACCURACY. Note that for a club-level player, this possibility is understandably highly UNLIKELY.
b) He has made a mistake or at the very least an inaccuracy that affords you some type of advantage (either big or small) PROVIDED you are competent enough to seize this advantage in a timely manner. This is quite common during theory deviations at club levels of play, though only strong opponents are consistently good at punishing these types of deviations.
c) He deviates not as a mistake/inaccuracy, but rather as a "do nothing" move such as pushing a h3/h6 pawn for no damn reason. Once again, while this move does not deteriorate the position, it still bears a cost in terms of time ... your opponent may literally get a free move which is quite vital in the opening stages when initiative matters a whole lot.
d) This deviation is psychological. He either wants to lure you to a unsound (at Master levels of play) line that is trappy (which he knows well) or will make you burn your clock during a timed game. Once again, against strong players, this is usually suicide unless the time controls are really short.
Your post posed a implicit question " What's the point of theory if that club player across the table will deviate within 3-4 moves?"
So with a) - d) being said, the advantage of "knowing the theory is"
i) When a deviation occurs, a light bulb goes on in your head alerting you to the possibility that there is possibly an advantage for you in this position. It's worth investing some time to prove his idea wrong, if you can. At club levels, this light bulb usually signifies that he played a mistake, inaccuracy or created a weakness worth exploiting.
ii) While your opponent burns his clock trying to "work out" the theory in a real game, you know it already and you can save your time for the rest of the game, which is an advantage inherent to time management. In games with strong players, you'll see them rattle out the initial moves because both of them know how important it is to conserve the clock for when they actually need it.
iii) You are aware of transpositional ideas, how one opening's line can transform into another opening's variation/line ... making your playing style a bit more flexible.
iv) In the case of psychological deviations and offbeat lines you don't know, you still have good opening principles and guidelines to navigate these waters and by knowing that your opponent is deviating in this unsound way, you will be extra careful about playing tactically safe but still solid chess.
H All,
Having recently taken up chess again after a number of years absence I have often wondered that set openings would only work towards winning a game if my opponent replied with the correct conventional move, but if my opponent made a move that did not follow the correct convention. So then I would have to move away from my intended opening moves to answer my opponents possible threat. Can anyone throw some light on this situation or am I missing something here?