Theoretical replies don't mean winning chances. KG was played long time ago in one to one matches, but in the top tournaments it wasn't played.
Most flank openings will limit your development and they give black time to develop. Most likely white won't castle on that side and it communicates quickly to black how to proceed. Does this mean anything at the under 2000 level? No, but if you want to get better you got to play what the big boys play or they won't invite you to the big boy table for dinner.
But even the "Big Boys" do not always play what is best.
2.c4 is clearly better than 2.Bf4, yet Carlsen has played the London.
2.Nf3 and 3.d4 is clearly better than 2.Nc3 and 3.g3 vs the Sicilian, yet Spassky, Smyslov, and myself play the closed regularly!
I believe it was Topalov against Kramnik, but might have been Shirov, but in 1999 at Linares, round 7 I want to say (of 14), the Cochrane Gambit was played - the game was a draw.
You need to understand the main lines, and will not succeed only knowing the London System or Bird's Opening, but you do not need to pigeon-hole yourself to main lines.
I've seen this phenomenon where some openings seem to be over/underrated take for instance the Sokolsky Opening, Bird's Opening, and the King's Gambit, the best theoretical reply by black equalizes in all these openings, yet The Sokolsky is called unsound and shit, the Bird Dubious while people call the King's Gambit sound and a viable option at the highest level of play, why is that?