Asking a few nearly trivial programming or other puzzles in the initial job listing or in a first phone screen really helps for two reasons:

1) it discourages lazy people and resume spammers. Most people want to cut and paste their resume and cover letter and be done. Someone solving your (even simple) puzzle means they might actually be interested in what you are doing.

2) You make sure they aren't completely incompetent jokers. They pass the trivial reject.