Wait, are we talking about replacing the USB port that can do an airdisk on the TC with a TB port, connecting a TB disk to the TC or are we talking about connecting to the TC with TB between the host and the TC itself ?
I think the point is having an Airport Extreme with a TB port instead of a Time Capsule.
Knowing that the drive inside TC is a WD Caviar Green (max burst data rate of 250 MB/s and with an average 98 MB/s), but also that any mechanical drive could face catastrophic failure, a single drive is not only a slow solution but also a dangerous one if you want to have a reliable system.
Sure right now the bottleneck would SEEM to be the ways to access your drive (theoretically 75MB/s max for WiFi n, and 3x125MB/s for the ethernet ports(which have a low overhead reaching 111MB/s in real life tests)).
But IF the device was able to concurrently respond to all the ports, the disk would be saturated (98 Vs. 450 total) even the USB 3.0 specification citing a realistic data rate of 400MB/s, just under the theoretical max demand that could be made to the Airport Extreme.
And this is now...
What then, when WiFi goes a lot faster in a couples of years?
Right now the 75MB/s are theoretical in a few year they might have become real use cases.
Somehow I believe the user might be OK with replacing a networking device every couple of years, not so with a functioning backup system.
So having a Airport Extreme with a TB port would cater to those how want to have a reliable RAID system, that would be future proof (rather have a system that ends up being a little under used than a system that become the bottleneck) if networking speeds improve.
P.S. one more case where it could prove usefull...
Your computer drive crashes, you change the drive and connect to the backup RAID directly benefiting from TB speeds for recovery. At current real world performance of 800MB/s against theoretical USB3 data rate of 625MB/s TB would still be 28% faster and against USB 3 specification 400MB/s you get 100% faster.
Of course there is also the fact that a number of people bought iMacs and MBPros that didn't came with USB 3 and it would be even more reason to want a AE with TB instead of USB, any USB.