Although audio and video are often thought of together as "A/V", they are truly different animals.
Audio and video have different data transaction requirements, and therefore each have their own storage needs. Because of the relatively small size and large number of files used in many audio applications, processing digital audio becomes very transaction intensive. By comparison, video applications usually require extremely high bandwidth due to the constant demand for a smaller number of larger files.
In multi-track audio there are generally numerous data files for each track, especially with punch-ins and edits. The rotational speed of the platters directly affects the number of audio tracks you can achieve on a single drive, because of the time taken for the drive to find and process the various files. Files are written to the sectors across the platter, and over the course of your project, these files become scattered, resulting in drive fragmentation. The heads move back and forth across the platters to write and read the scattered data.
Because of the time it takes for the data to pass underneath the heads, drives with slower spindle speeds take longer to access each file than faster drives. Audio applications are very demanding on drives because small delays in the delivery of requested files can result in playback errors. Faster spindle speeds yield more audio tracks with more edits. A 7,200 RPM drive is the minimum required for average sessions at today's sample rates.
On average, Video files are much larger than audio files. For example, if you were to capture half an hour of DV quality footage you would take up over 6GB of disk space. If you are working with uncompressed quality video, you could expect to use over 40GB for half an hour. NLEs trying to record and playback these files need sufficient bandwidth to do the job. If the data is not transferred quickly enough for the application to process it, frames will be dropped.
Bandwidth refers to the amount of data that can be transmitted in a fixed amount of time, and often is described in megabytes per second (MB/sec). Bandwidth is more important for video than spindle speed because of the way NLEs access files. NLEs create large files comprised of detailed pixel/color information. These files require a big pipe to flow through at speeds fast enough for the NLE to process. Realizing relatively high bandwidths can be done with consistent use of high speed interfaces such as FireWire, and SCSI.
In addition, individual drives can be configured in a RAID, which spreads the I/O load over several drives. With a relatively small number of large files, disk drive heads do not have to work as hard to find scattered files, and for theses applications, bandwidth is more important than spindle speed.