Let's be clear here ... there are 3 interpretations of "Avoiding theory"
Case 1. I am going to play something I pulled out of my b#tt and I'm making it up as I go along. I will say "I'm avoiding theory" but I am hopefully using sound opening guidelines/principles and playing according to the pawn structure in front of me. The downside is that am just going to burn my clock figuring things out on the board and if this happens to be theory (that I don't know!), my opponent will gain on time and have more to use in the middlegame/endgame.
Case 2. I am going to play a rare/unsound (for Master-level) opening or line that you probably have not heard of but I know quite well so that I can either make you burn your clock at the worst case or have you walk into a tactical minefield as a best case. The downside is that if you know this line or how to bust it, I am in trouble ... but that's a risk I will take.
Case 3. I will play a lesser known/fashionable line of a popular opening (say the Monte Carlo version of the French Exchange) counting on the fact that not many people book it up..... so once again, I will make you burn your clock to find accurate moves or have you walk into dangerous territory that I know quite well. Unlike case 2) where the line is proven to be unsound at Master level, the downside here is that I may get equality at best with this not-so-popular line but atleast we're still playing in my backyard (if I'm still in my book) or "chess" as they like to call it.
I've seen the majority of players who continuously improve fall into case 3 and yes, they are "avoiding theory" but not in the way you'd normally think.
Notice that I'm bringing the clock into this a lot ... with most weekend tourneys in the U.S gravitating towards a G/30 or G/45 time-control, you'd be stupid not to consider booking up "something (Case 2 or 3) " if you want to stand a chance against strong players.
I know a lot of people, and I see posts here as well, who play offbeat openings.
When I ask them why they explain "I want to avoid theory." or "This way we're playing chess from move one."
I don't understand that. Wouldn't you want to learn some theory with the intention of improving more?
Learning opening theory (at least to me) doesn't just mean learning reams of variations. It means learning where the pieces go in a particular opening and why. Sure, if you're a Super GM you're going to need to know every line of every opening you might ever face 20+ moves deep. But if you're a club player you need to know a few lines 10-15 moves deep, and more lines 8-10 moves deep, and in general just know the ideas behind the openings.
I'm curious to hear from both sides on this one...