Some libraries like libx264 are built for multiple cores especially when used with ffmpeg.
I know them quite well... I've done some minor dev work on them. But with multi-threading, you get diminishing returns and can't fully max-out all the cores, no matter how much you want to, you'll have some idle CPU time on some cores... increasing amounts as you increase the core count.
This is why Firewire and eSATA and Thunderbolt are preferred. All of them have independent chips which handle the work so the CPU doesn't have to do it.
A completely specious argument... The overhead of a USB transfer doesn't drag-down a modern CPU. Take just a few dollars of the money you'd pay for Firewire versions of your gear, and use it to get a slightly higher-end CPU. Now the overhead of USB is covered, you've saved some money, and you're better-off all-around.
Firewire is around because of legacy DV cameras. If Firewire was any good, eSATA wouldn't have ever appeared. But it's still around mostly because it had several years' head-start on USB3.
Thunderbolt is only a thing because Apple has a dog in the fight, so they insist on forcing it on their users like Firewire before it, or PPC processors before it, or now with H.264 and blocking Flash and WebM, or a million other examples.
I've said it is a poor choice for realtime work
Except I already said USB isn't good for real-time... But with most use cases USB can easily do faster-than-real-time, which eliminates the issue.
Your example would be to argue that Toyota Camry is fine for all home improvement because they are everywhere.
No, my argument would be that you've so far been unable to name even ONE single case, where your Toyota Camry has been insufficient for the task...