[roygdavis]

And then some time later someone adds a nullable boolean to a column in your database and that null (3rd) value ends up representing different things.

[karmakaze]

Even worse, MySQL typically uses TINYINT a 1-byte integer to represent a boolean value, so you could have NULL, FALSE(0), TRUE(1), and 254 other 'TRUE' values between -128..127 (not counting the 0..1 canonical values).