You hear a lot of advertisement for hand grips, finger weights, etc to improve hand and finger strength. What's wrong with the idea of playing your guitar as much as possible to achieve those results?

If you exercise your hand and fingers by actually playing your guitar (e.g., making barre chords, playing songs, scales, what-have-you), you'll get the strength, improve your playing, and save a few bucks on equipment whose only purpose is to build muscles.