It's all a matter of purpose.
eSATA on it's own doesn't really offer any direct benefits over firewire or USB2.
However eSATA is designed primarily for hard drives or other ATA compliant devices. USB and Firewire are designed for many many uses. So logic would say if you've got an eSATA port available, you may as well use it because it's not going to be used for much else, whereas a USB or firewire port could be.
Also bear in mind firewire and USB are daisy chain technologies. This meaning that your 400 or 480mbit/s is shared among all devices connected to a root hub.
So if you were to have a USB hard drive on a root hub along with a 300mbit WIFI "n" adaptor your hdd and 300mbit wifi would share the same 480mbit.
Conversely with eSATA the only thing it's likely to share bandwidth with is your other HDDs. Unless in raid you're not likely to read from more than one disk at a time in such a way that'll cause bottlenecks.
A quick note about SSDs. They are not "faster" than magnetic storage. They have a very low random access time. However, as you will find by running HDTach on a Flash drive, the sustained read / write speeds are lacking way, way behind that of magnetic storage and RAID setups. This is great for sticking your virtual memory on and for server based OS drives and such where things like registry are rarely referenced and written sequentially, but for the average Joe who wants to copy files to a DVD, install programs, play games etc SSDs will seem slower loading/saving things than a well kept magnetic drive.