I guess an extra queen is not necessary.
Almost allways the original queen was already removed, or at least one of the rooks, that you can turn upside down with no problem.
If you still have both rooks and the original queen when about to promote a pawn... well, the situation is probably terrible to your opponent and they usually resign before that anyway.
According to Wikipedia, promotion occurs in about 1.5% of games and in about 97% of the cases it's a promotion to a queen.
I'm wondering, does anyone have any statistics on how often, out of those cases, did the promoting player still have the 'original' queen on the board? In other words, many chess sets have 4 queens - how often are those actually required?