At desktop, you can easily press NumLock with one finger.
At laptop, you'll get bunch of cryptic words (like Kevin's ones), swear aloud, then use BOTH your hands to turn NumLock off, then Backspace, then writing everything again.
So, bad idea.
Good idea, I work alot with numpad and its annoying when I'm typing in some data, to discover a few secs later when I look at the screen that all I pressed were some arrow keys and the page navigation.
It would be nice if it was an option. I know for my desktop I like it on when I start my machine up, but for my laptop I can't use the keyboard until the NumLock is disabled.