First of all, that's RJ-45. And it wouldn't be an adapter, it'd be a port, or a jack, if it were a built in RJ-45. Second, except for the very early Series2 units, those with model numbers starting with '1', the USB ports are 2.0.

When the Series2 was developed Ethernet was a more expensive option, and USB was also anticipated to be used for other things - at the time TiVo was talking about gaming, media devices, etc. Things changed over time. The USB system works, and it handles wired or WiFi. The CableCARD box is expected to have a 10/100baseT port, as well as USB for WiFi, as network performance will be more important with the larger HD files.