Multiple partitions means you won't have to defrag as much/long than with a single partition and it's more organized.
Only thing I don't like about partitioning is that when you drag a file from one partition to another it's that it gets copied & not moved over, unless you use menus or something else more elaborate plus you have to change program install paths when you install something since most programs like to install into the "Program Files" directory on your main partition.