adb shell mount command: documentation?
On some devices/ROMs most mount points seem to be defined in a special config file called /etc/fstab. In this case, you can ommit the device information and options, as they can be automatically determinded.

However, other devices/ROMs use init scripts to create the mounts, and have the special options defined there (CM7.2 on my Motorola Milestone seems to be one example for this). If you omit the device information here, you will simply get an error.

So which variant to take very much depends on the device/ROM. Safest way is to first check how the file system in question is mounted (e.g. using mount |grep system to show it for the /system mount point), and then adapt this information for your use.