So the Jaguarboard is a very nice device, with lots of CPU power in very small size, which is great. However, it has neither USB 3.0 super-speed, nor gigabit ethernet. As such, it is extremely constrained in I/O bandwidth.

Let's say you want to use it as NAS. You can attach some USB 2.0 hard disk, but then the 100-base-tx ethernet is too slow for that.

Let's say you want to use it for interfacing a popular SDR board like USRP B2xx or BladeRF. However, the USB 2.0 is too slow for full sample rate/bandwidth. And even if the USB 2.0 was sufficient, again the Ethernet is what blocks the use case, as you will never get the data off board.

I can think of plenty of applications where some high-speed interface (with or without intermediate processing) between USB and Ethernet, but the Ethernet at 100 MBps is just too slow, even for USB 2.0 HS at 480 MBps.

Yes, you are right. The USB and NIC is the bottleneck of many applications, so we have Z8300 with USB3.0 and GBEintegrated in next gen.