- Those who want more storage without external latency
- Those who want more than one or two monitors
- Those who want addon cards.
Thunderbolt is capable of solving all of these problems, and in fact there's already market solutions for them. The problem is that the enclosures cost more than the Mac itself, and that's not including the hardware you want to put into them.
Thunderbolt drive enclosures and RAID systems are already starting to fall in price and will continue to fall as TB gains marketshare, but the PCIe cages are still STUPID expensive. If Apple were to release their own bus cage for people to add PCIe cards to their macs at a sane price point, suddenly the MacPro stops being the only Mac with that capability.
A chain-able TB display adapter would also go a LONG way to expanding the Mac's capabilities. One DisplayPort output per bus just doesn't cut it for some power users, and the only alternative is significantly slower USB adapters.