If you are going to use anything as a server nowadays you need Gigabit Ethernet except maybe for a print server. The Mac Mini G4s do not really meet the needs of most servers nowadays. Their G4 processors are rather slow, they are limited to 10/100, and do not offer more than one internal hard drive. A PowerMac is better in every way as a server with the exception of power consumption.
This!
I like Erik's idea with using an ibook, if one wants to stay PowerPC, it has a lower power consumption and the prices have come down to PowerMac niveau, too.
Also re Gigabit ethernet, I heard that the onboard GE is very slow, compared to afterwards installed GE-PCI-cards. (There was a tip for a cheap card somewhere here in the forum.)
On the OPs question. I use my PowerMac, because it is the only Mac that can get detailed SMART info of Hard drives and I am a sizzy so I like to check SMART from time to time. The PowerMacs can read the by the Drive itself stored SMART data on the onboard IDE port (via IDE-SATA adapter) or on my Macally SATA card (Sonnet only supports SMART on the eSATA or 4xport cards).
Also, I run some older apps, that I don't like to invest time in searching for a newer alternative or buying newer generations of it. (I lately got me an Intel-MacMini, that I picked with the possibility to use older binary software in mind, the number of Apps needed on the G4 is now narrowing and what will be left for the G4, though, is SMART and OS 9 games.)