I prefer to have my root volumes separated from the data volumes.  For one, this helps to keep the root volume from becoming 100% full.  In the even a root volumes becomes 100% full, a filer may simply reboot or all out panic.  Before anyone tells me how remote that chance is, I have dealt with those consequences already.  It is more of an annoyance then a real problem but the separation of root volumes provides me with one less thing to worry about.

I must agree with all of the others reasons as well as they were so well put.  If one were inclined, the cost is easily justifiable in larger shops but I will admit in smaller installations such justification may be harder to come by.

Number of disks required to store a core dump

When the filer creates a core dump, it saves the contents of the memory and NVRAM to a set of fixed-sized areas at the beginning of all the disks. Therefore, you must make sure that the filer has enough
disks to create a complete core dump for a given size of memory and NVRAM.

For DOT version 6.0.1 here are NetApp recommendations for the minimum number of disks that must be installed for a filer to successfully perform a core dump is shown in the following table.
 
Memory     NVRAM     4-GB disks         9-GB disks       18-GB disks     36-GB disks       72-GB disks
512MB 32MB 6  3  2  2  1
1GB  32MB  12  6  3  3  1
3GB  128MB  n/a  18  9  7  2

as pulled directly from the DOT 6.0.1R3 Start Here document, Chapter 2: Information for All Users, System requirements for this release of Data ONTAP.
 

neil lehrer wrote:

are there any particular advantages or disadvantages to just having ontap on
vol0 and data on other volumes?
--

regards