Since 1993, that's what I've been missing from my mine sweeper game. All these years I couldn't understand why I could never finish the game. Hahaha! :)
There are actually some expert players who intentionally play without the flags as an extra challenge. They have to remember (or re-deduce) where the mines are. The game is considered won whenever the number of unexposed squares, whether explicitly flagged or not, or equal to the total number of mines.