When to run eseutil /d on Exchange 2007 databases?
Have your boss read this article and then decide whether it's appropriate or not.
Whitespace is not inherently a horrible thing. It's a completely natural thing to have happen over the course of natural MS database usage. That said, I would only look at removing the whitespace if I was having issues with backups completing in a timely fashion, otherwise I don't care, and I'd rather not take a chance of causing an issue with the database.
The only unconditionally justified use of defragmentation is when it is done as a "cleanup" after a database repair operation in my opinion. In most other cases, you simply should not care. The mail database tends to rather grow than shrink over time, so database areas marked as "free" today are going to be utilized shortly in the future.
Unless you have a very specific situation where you desperately need to reclaim the space, I would not defragment - especially since it requires downtime for the to-be-defragmented database.
The only time to run offline defrags is when you have a large enough quanity of whitespace that will not be reused. Offfline defrags are not a normal nmaintenance item. How much is large enough? That depends on your infrastructure and the size of the databases. If you are recovering serveral terabytes of space it's probably time to run a defrag. If you are recovering several gigabytes of sapce - it may not be time. Additionally instead of running defrag, create a new database and move the mailboxes to the new database. This will be less impactful than an offline defrag.