I do it all the time too. I have a Boss LS2 line switcher so it all works pretty seamlessly. Takes about 10 secs to do a swap. Most gigs with the current band I'll use my fretless Wal, fretted Yamaha BEX 4 and NS CR4 EUB (pizz and arco). Sadly, on one song, I play the first section on the EUB and the second section on the Yamaha (I'll have the Yamaha hanging on its strap while I'm playing the EUB).
Whether the punters really notice the difference is of course a moot point - although actually I'm pretty sure when I swap from, say, fretted with flats played with a pick to round-wounds on the fretless played finger-style, anybody actually listening would probably be aware that something's changed. I think it's more complex than that though. The sounds we get playing bass fundamentally influence the the way we play a song. It's not just about the audience hearing (or not) different bass guitar tones, it's about the sound of the whole band and the performance.
P