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Because as it is, Firewire 400 is already faster than USB2 (up to 70% faster due to the architecture). People doing graphics or video work would need even better, Firewire 800. It takes an inordinate amount of time to transfer multi-gigabyte files to my 500 GB USB2 external drive. My next one will be Firewire 400 or 800, and I'll barely notice the difference between an internal or external drive.
Might want to re-consider Firewire. eSATA with 3gb/sec compared to 400/800MB/sec of Firewire is like the difference between old ATA33 drives and SATAII 300 drives.
First, you need to get your numbers right. eSATA is 3 Gb/s (300 MB/s) and Firewire 800 is ~800 Mb/s (~100 MB/s). eSATA is three times as fast on paper. But one hard drive can only read or write so fast, so in reality they're a little faster (maybe 20%), not three times as fast. However, eSATA would be a great choice to connect to a large external RAID, where all the drives together could actually saturate the eSATA line.
Firewire is far more flexible for my needs. I can daisy-chain lots of devices, including my digital video camera, and it provides power for its devices (IOW, no power brick required for the hard drive). eSATA provides no power and is one device per line (unless you buy a port multiplier). Firewire also lets me have a longer cable -- good for the camera, and it's easy on the processor since it's a peer-to-peer system (devices can actually talk to each other without needing the computer).