If you think of a keyboard as playing only notes, four or five octaves may be sufficient. However, many virtual instruments (e.g., FXpansion Geist, Native Instruments Kontakt, EastWest's Play engine, etc.) use MIDI keys not only to play specific notes but also to trigger articulations or variations on a basic sound. If your main USB MIDI controller doesn't have enough notes, no worries—trade it in for that deluxe 88-note weighted keyboard you've always wanted (hey, you only live once). But if you lack the space or finances, add a second USB MIDI controller for doing switching—even if it's just something like a little Korg plastic keyboard designed for mobile applications. Your sequencer probably won't be able to merge incoming MIDI streams, but no worries there either: MIDI Solutions's Merge will merge two data streams to a single output. There are also several DIY circuits for MIDI mergers on the web.