Posted By:
Joe D.I most definitely prefer writing on a postcard and love when they are used postally.
If the writing has baseball content - then its an even bigger plus.
A pristine, unused postcard is kind of boring to me.
Give me one where the person calls it the "best team he is ever seen", or "the best bet in town", or "I just went to Thursday's game". Stuff like that - can't be beat.
My personal preference aside, writing knocks the grade down to an SGC 10 or SGC A (depending on their mood) - so the boring postcards do bring a higher value 
Edit to say:
Look at Martin's postcard. Postally used just before the World Series between the Mighty Cubs and the Hitless Wonder White Sox. Yet the person states, "These are what all of Chicago is raving about"... and it is the Hitless Wonders Postcard - before they won the series against the mightier opponent! How anyone would prefer that to be blank is beyond me. Awesome baseball content.