Uhhh... odd. USB 1.x tops out at 12 Mbps with one device ONLY being able to take a maximum of 800 Kbps at a time.
So while a 10 Mbps ethernet card is a good match (actual speed will be less than the theoretical max 10 Mbps anyway), 100 Mbps... what is the point? The bottleneck will still be on your system's ethernet device since it will have to throttle down and request resending of the ethernet frames.
Which, if you're on a hub or the same collision domain as other systems will probably degrade the whole network though perhaps not as much as having a 10 Mbps ethernet device attached.
Unless I'm missing something, this adaptor only allows you to plug an ethernet device into the USB port? Granted, I'd bet this is all for home use anyway and no one will notice any real performance or network issues on a small scale, but still...
"BTW, we also support Gigabit ethernet via our USB adaptor!"