I haven't got any experience with Mikrotik's virtualization and USB ports, so the question would be is it possible to access that USB port from the guest host running on RouterOS? 32MB of RAM seems a little low, but if it was possible to actually run any opensource linux-based OS (for example OpenWRT), this would become an ultimate router for simple home use. I mean network storage with samba sharing, torrent downloader and so on.
There are many ready-to-use devices for such purpose in market, but having possibility to assemble such box of Mikrotik hardware would be really nice. In tandem with an external hard drive this RB would be pretty attractive solution. Just a thought though