In games, how often is the screen actually making such drastic changes?

Constantly.

Not really actually. Sure, the content is changing, but the game is not sitting there flashing back and forth between black and white as quickly as it can. Take Quake 3 for example, on an indoor map. Your running down a typical hallway, and do that 180 degree flip. The walls are mostly the same colors, so the LCD doesn't have to change the color that much Probably the most drastic change will be any actual players, and still it's not going to be a light show every other frame. Thus, the 25ms rating of my monitor means squat in this situation, beyond giving me the absolute worst time I will ever see. As long as VSync is enabled, limiting the game to 60 fps, there shouldn't be any issues, since the lower response time panels out now can easially come very close to 60fps of black and white flashing.

I highly doubt any gamer here would have a problem gaming on the more modern 16ms panels. Anyone who says they do, I liken to the people that say they have to have FLAC over 320kbit MP3, while listening in a normal car.