I agree with the RME comments, though the PCI slot issue is a major drawback. For me though, it is less about using multiple RMEs than adding on accessories, and flexibility.
For example:
*24/96 capable -- not as much as a big deal today, as more are coming out, but many firewire interfaces are still only 48.
*ADAT - Can add a 4-8 channel pre like the Digimax - with the option of using it at 16/48 or 24/96.
*Use with a laptop
*Switchable power - use anywhere in the world or with batteries
*Only 1/2 rack size - so can expand (e.g. pre's, comps, more i/o) without taking alot of space
*Recently announced they are coming out with 4 and 8 input mic pres to use with the rme
*Midi interface
The PCI is a drawback and non bus power on laptops is another, but otherwise, I find the RME very "expandable"