Presumably BIOS floppy emulation is technically possible using this technique, however no known implementation exists. The USB transport protocol is common, but many devices e.g. scanners and printers still use proprietary formats. Having USB support doesn't mean that all USB devices will automatically work. In reality you get 8 Mbps at best, with 6 Mbps being a typical result. Every joystick movement impacts that rate noticably.
Transactions have big headers.
Idle devices still consume bandwidth. It's not a chip or protocol limitation, but rather a power consideration. USB ports must be able to supply up to 500mA to devices, although the devices must initially limit themselves to 100mA when plugged in. This is a substantial amount of power, thus USB hubs commonly have low port counts. A four port hub requires a 2.5 amp power supply.