People often play:
- d4 Nf6 2. c4 e6 3. Nf3 b6 4. g3
And allow both the Queen's Indian Defense:
4... Ba6 OR 4... Bb7
And the Catalan:
4... d5 5. Bg2
Why not play:
- g3
and transpose into Catalan, instead of learning theory of both the Catalan and Queen's Indian Defense?
White can't force black to go for a setup with ...d5, so it's not a given that 3.g3 will end up in the Catalan.
In particular there is 3...c5, trying to transpose to a Benoni where the g3 variations aren't the ones with the most dangerous reputation.
The issue is in committing to 3.g3 a move earlier. Then if Black plays 3...d5, White must play the Catalan (with 3.Nf3, he still had the option to play a different line in the Queen's Gambit). Or take 3...c5 - against 3.Nf3, White has some edge in a few different lines. But against 3.g3 Black should be pretty close to equal, since as @RemcoGerlich mentioned, the Fianchetto variation isn't White's best try against the Benoni.
Of course, some people still play 3.g3 anyway, since they're fine playing with a fianchetto in all these lines. I think it's what Avrukh recommends in his books. It's just not everyone's cup of tea.
Shortly:
I am an experienced Catalan player and choose 3.g3 order exactly to disallow Queen's Indian. Considering my knowledge and practice I can say that most of White players move 3. g3 for the same reasons.
Little more deeper:
Main issue here is that at the point after 1. d4 Nf6 2. c4 e6 White doesn't really know if Black selects some classic structures (like Queen's Indian or Nimzo) or more sharp (Benoni, Benko). By сhoosing early 3. g3 order White limits Black from Queen's Indian, but by this price allows Black to switch for more preferable options in, for example, Benoni: 3. ... c5 4. d5 ed 5. cd d6 (or 5. ... b5!?), where g3, Bg2 system is relatively harmless for Black.
Thus, one shouldn't treat 3. g3 as the move that just narrows down Black's options. It has own drawbacks. I can speculate that if you are preparing as White to more positional-styled opponent or Queen's Indian player it is better to consider 3. g3. If your main weapon against Benoni is the system other than Fianchetto (g3, Bg2) you probably need to check some QI lines and go 3. Nf3, it will be more sound in this case.
The QID isnt really anything to worry about. Most white players are more worried about the NID or the KID and are probably happy to see a QID. White scores 54% and SF gives .37 to white so white is able to maintain an advantage
The Catalan doesn't really score better and is only .11 according to SF. I don't really see the point of learning an entire new opening if it doesn't give better results.
Transpositions- I don't play these openings but I'm sure there are some tricky transpositions.
So, you're saying you should learn the queens gambit AND the Catalan and then learn all the transpositions and sidelines etc. What are you gaining by doing that? It just seems like a lot of extra work for no reason to me.
