Journaled format no-no on external FireWire drive

From OS X Mountain Lion the missing manual, page 792: "You can use the journaled format on your main hard drive, but not on your external Firewire drive - or vise versa." Why not? Is this new for OS 10.8? The journaled format on my old eMac, with OS 10.4.11, and external FireWire hard drives seems to have worked well for many years. Must I reformat these externals for use with my new iMac, with OS 10.8?
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