Like Kev: the pros are lighter weight and greater strength. Another pro is the ability to adjust the headset with a single allen key.

The biggest con is that you cannot raise and lower the stem easily. The spacers do give you some adjustability up and down, however. If you need to lower your handlebars, take some spacers out from under the stem and put them on top. To raise the stem, do the opposite.

A lot of bikes come new with no spacers on top of the stem. For my 2 cents worth, this is a bad idea, unless you have been very carefully and well fitted to the bike.