What a load of absolute claptrap. For audio production expansion can be vital. If you want to use ProTools with any complexity, or want to use the full range of audio hardware, you want the ability to add connectors the Mac doesn't have, or doesn't have enough off. And a FireWire hub just isn't suitable for some Audio Hardware situations. (For some things yes, but for others you really don't want to have it off a hub, as I know of some Audio Devices that can easily saturate a FireWire 800 connector, and would be awful over a FireWire hub). I can't think of a single even prosumer music artist or producer who would want to be limited to just the few FireWire and USB connectors a Mini provides. Once you go past GarageBand, you really want to not be limited to the very limited expansion a Mini has.
Yet again, a $500 PowerMac G5 or a Mac Mini with thousands of dollars worth of equipment doesn't make sense.
Besides, I pointed out that the next Mac Mini due in a few weeks is going to carry Thunderbolt, which is basically external PCIe x4 - and has 12.5 times the bandwidth of FireWire800.
If you're between a PowerMac G5 and a Mac Mini, you're not likely to spend millions on equipment anyways. If so, a new Mac Mini is always the wiser choice, as it runs Lion and the latest builds of audio software - plus being faster and having future-proof Thunderbolt.
For the current-gen Mac Mini, you could daisy chain your FireWire audio hardware and hook up your hard drives via USB, as the hub is intended for devices that come with one FW800 port only - which should be mostly the case for hard drives, but not audio hardware. This should be sufficient for everyone who considers a Mac Mini for music production in the first place. Most prosumer audio interfaces still ship with FW400 anyways, MIDI interfaces with USB. Controllers, even with motor faders, mostly hook up via MIDI or FW400.
But the costs for a prosumer audio interface, some MIDI interfaces and a controller are somewhere between the cost of a 8-core and a 12-core Mac Pro, thus people buy the next best thing and a Quad- or Six-Core Mac Pro.
If you prefer a PowerMac G5 over a new Mac Mini, I'm perfectly fine with that. Have it you way, but don't complain that you are i.e. restricted to Logic 9.0.2 and probably end up with Logic 8, because Logic 9 does't run that well on PPC at all. Or can't run the somewhere in the future upcoming Logic X. Or, as already stated, Cubase 6.
You basically put up everything against "I might spend $5.000 on equipment in the future and there is a minor chance that I need more than one FW800 port", which dissolves itself into "I might spend $4000 on equipment and $1000 on a used Mac Pro in the future" and/or "My Mac Mini has Thunderbolt".