I've done gigs using a pignose mic'd into the PA or even a spare channel input on the bassist's amp before now - I took to carrying one years ago as an "emergency amp" in case my main amp decided to die at the wrong time. It's also a surprisingly good harmonica amplifier and I know several people who use them for that.
By the way Beepster, if you're looking for a domestic-premises friendly amp 10 watts is about 10 times the power you'll need. Using the same cabinet, a 5 watt amp cranked flat out is about half the volume (in dB) of 50 watts. I've an Epiphone Valve Junior (with Phillips valves and a Jensen speaker) and in the house I rarely open it up more than about a third, at which point it's already pushing 85dBA at a meter.