The payment of a dividend is a neutral event, since the stock price will open lower, by the dividend amount, on the ex-date.
However, stocks are traded by human beings. And human beings like to "get in" before the dividend. A lot of newbies trading stocks don't understand how the ex-date works. I swear, on Yahoo forums, *every* dividend cycle, you can see people asking "when must I buy to get the dividend??"
Therefore, stocks which pay a decent dividend (I'd say over 2.5%) show some slight upward pressure before the ex-date.
It can be profitable to buy a few weeks before the ex-date, and sell before ever getting the dividend, if you scalped enough on the stock...
The reverse is true as well, stocks can drop by more than the dividend amount on the ex-date, if it is weak, because people were "waiting around" for the dividend. Short those!
So, think of it kind of like the "stock split" effect. There is no mathematical benefit in buying a stock before a split. But people still get excited about splits, ("Ooooo! I have twice the shares!") and will buy stocks before the split date. It's just "buzz" for the crowd...
I wouldn't mind at all seeing dividends become tax free... Not so much to GET the dividends, as much as to see the general public chase them. It would generate more interest in div-paying stocks, and make them easier to swing trade based upon this crowd effect.