Unless you have a true, overriding reason, put only one partition on a disk drive. The arguments often put forward in support of multiple partitions don't hold water. Let's analyze a few.

I want to have a separate data partition, so I can back that up more often.

You don't have to have separate partitions for that. Folders can serve the same purpose better. You could have one data folder and put all data into subfolders inside it. Alternatively, backup programs can be set up such that they back up a list of folders, such that you don't have to enter them every time.

Multiple partitions reduce disk fragmentation.

Actually, they don't. Why should they?

A related point is that, on a busy disk, you may want to use a defragmenter, which solves the fragmentation problem anyway. Have a look at the disk fragmentation article too.

Multiple partitions increase performance.

The reason why partitions often decrease, rather than increase, performance is that they increase disk head travel. An example would be a disk split into two large partitions that are partly empty, with the files being in the beginning of the partition. Every time a file in the other partition is accessed, the disk heads have to travel across the empty space in the first partition.

The worst one can do is to put the page file (virtual memory) into a separate partition at the end of the disk. If the page file is actually used, this would nearly maximize the disk head travel times.

Conversely, on a one-partition hard disk that is occasionally defragmented, you will have all files in the beginning of the hard disk, which minimizes disk head travel. Some defragmenters and Windows' own prefetch mechanism additionally move frequently used files or files used at boot time together to minimize disk head travel.

I can manage disk space better.

The truth is that you'd only create the necessity to manage disk space in the first place. A disadvantage of partitions is that they split the most valuable asset of your hard disk—its free space. Since you cannot determine in advance exactly how much space you will need in each partition, as the disk fills up, one partition will fill up first, while another will still have free space. Then you have to waste your time to resize the partitions.

All that said, there are some good, overriding reasons to have more than one partition on a hard disk. One of them is that you want to have two or more operating system installations on the drive. You cannot, for example, install a second Windows into a partition that already has one, because of folder name conflicts.