Not just fanboy arguments. You could boot off them just like a SCSI device. This was not possible with USB drives at the time (and is still a messy affair). The performance was very different too.
Firewire/i.Link/IEEE 1394 was also the dominant standard for video streaming. Nobody even tried using USB for that AFAIK.
Light Peak/Thunderbolt replaced both of these uses for 1394, so I think it's fair to say it is a true replacement for it.
Bootable USB may be messy to prepare, but on the user end it largely Just Works.
As with many hardware-vs-software things (e.g. RAID controllers), Firewire performance was originally much better but USB caught up as processors got faster.
A standard for USB video streaming eventually emerged (at least, I have that option in my kernel), but it's fair to say it was much less popular than 1394. Mostly camcorders (at least consumer ones) became digital themselves, storing data on a filesystem and presenting themselves as a mass storage device.
Firewire/i.Link/IEEE 1394 was also the dominant standard for video streaming. Nobody even tried using USB for that AFAIK.
Light Peak/Thunderbolt replaced both of these uses for 1394, so I think it's fair to say it is a true replacement for it.