Lets suppose we had the following auction in 2/1:
1S-2H-2S-3S (but the issues are the same when-ever you just agreed a major at the 3 level)
A. (Most Important) If you are not interested in Slam, and Neither is Partner, you do not want to help the opponents with an opening lead by cuebidding in a minor. This is a major issue, especially at MPs, but even at IMPS. Playing Serious 3N the auction might go: 1S-2H-2S-3S-4D(not serious)-4S and they lead a club....
Playing Not Serious it Goes: 1S-2H-2S-3S-3N(Not a great hand, but not hopeless for slam either)-4S and they have no idea which minor to lead.
B. (Less Important) The auction Tempos Better playing Non-Serious 3N, since its more likely that the good hand will be able to Q-bid 4C than the bad hand, saving you room for more 4 level Q-bidding
C. (Least Important) If you have an accident and your partnership has a misunderstanding about whether 3N is Serious or not, passing it is less of a disaster when it was non-serious, since, at least, you are not missing slam, and your side is less likely to be control rich and more likely to have the Q's and J's that make 3N a reasonable alternative contract to 4M. Although, its probably not your best contract in either case.
Anyway, thats my 3 bits.

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