You can probably also access applications across partitions but I would not advise it.
This was badly stated. I should have said:
It is normally OK to access applications across OSs but this can cause problems if the OSs are different versions or, in a few cases, if the applications are badly written and make assumptions about their location. Many applications assume any preferences or data they use reside in locations relative to the user's Home folder and this aspect may require some thought.
Occasionally the OS does its own thing and uses an application on the wrong partition. This can be annoying but I have never known it to result serious data loss.
It is technically possible to have different partitions for OS, applications and user data (Home folders) etc but the average user would not welcome the complication which this can cause.
My main 120GB disk has 8 volumes. Some with different fully functioning OSs and some with various modified OSs and installers. In addition to this I have several 750 GB disks for backup.
One nice thing about an extra bootable partition is that it can be used to repair another partition as long as Disk Utility is compatible with it. This can be a useful alternative to using the installer disk or AppleJack. Some recent OSs have hidden partitions with such facilities.