Defrag is less necessary on XP systems. But still useful.
Frequent Defrag does not help system speed that much, unless you do a lot of program installing and removing, or you use programs with large databases that get updated alot. How much it helps depends on how you use the system. If you are getting ready to install or remove several programs or update a lot of files, then it is useful to do a scandisk and defrag both before and after.
Your expert may be concerned about disk read/write errors showing up during a Defrag and causing problems, or file system errors causing problems. This is why you should at least run ScanDisk first in normal mode, and if it has not been ran for several months, run ScanDisk in Thorough mode (takes hours) before doing the Defrag.
The second concern, if there are a lot of file system errors and you let ScanDisk fix them, it can corrupt files that are otherwise useable.
But if you run ScanDisk and Defrag weekly, these types of errors should not build up to the point that they can not be safely fixed automatically by Scandisk. It is often useful to run the Disk CleanUp utility also, after running ScanDisk, as it cleans out some temporary files and allows you to run Defrag quicker.
The Microsoft Maintance Wizard sets up ScanDisk, Disk CleanUp and Defrag as scheduled tasks. So that is a clue that you really need to run all three programs. However, it is best to run them manually when you have the time and not use the scheduled task.