I generally don't see much value in an author spending his precious time signing books at libraries or bookstores... but do think that ANY signature adds value. I never give out a copy of my book without a not on the title page.
Of course, Potter takes this to a whole 'nother level...
2theadvocate.com: Entertainment - Livingston library gets signed Harry Potter book 08/14/05: "'It was a contest through Scholastic,' said Youth Services Librarian Stacie Barron. 'I just had to submit the name and address of the library.'
Scholastic had originally intended to give just one signed copy to an American library that was named the winner in the sweepstakes (that was West Asheville Branch Library in Asheville, N.C.). But, according to a company news release, the response was 'overwhelming.'
More than 9,000 libraries nationwide entered the contest. So Scholastic decided to name one winner in each state. The Livingston Parish Main Library in the town of Livingston was the winner for Louisiana."