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These 2 observations look to me contradictory. I also do not understand the comment about MP.
They are not.
You gain on 5-3-3-2's hands if you can bid 2C with them but if you bid 2C with them you lose with hand with clubs (or Gazilli hands). There is a trade-off there.
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It is often difficult to defend one notrump. Nowhere is declarer's advantage higher, when comparing single to double dummy play.
One day I pulled all the hands from vugraph where Meckwell/Lauria-Versace/Bocchi-Duboin and few others passed 1NT there. I went through all of them. I also played a lot of hands online and I was paying attention to this issue since forever. I am very confident about my judgement on this one because I did more research on it than probably anybody else. Just because 1NT tends to be a good contract at MP's in many situations doesn't mean it tends be a good contract after say 1S - 1NT. It isn'. By bidding on with 5-3-3-2's you get much better contracts.
I don't have as strong view when it comes to 1H - 1NT. I think passing here might well be better with 5-3-3-2's and 4-5-2-2's.
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2S on 5-2 is usually better than 1NT at matchpoints. There is also huge gain from 8-9 trumps heart contracts which you miss often if you pass with 5-3-3-2 after 1S opening, being able to play 2/3m if responder have 6+of them is another huge bonus.