I use a couple of tunings on my twelve string when I play, and I also use a capo. However, whilst I find that I can tune and re-tune at home quickly and with few troubles, when I've got people looking at me ..... I guess its nerves but suspect that I'm kind of use to the acoustics at home and so get a bit thrown by different ones in different halls. I'd guess that the need for another guitar is dependent upon the kind of gig you play. Go round to a gentle and informal gathering with loads of guitars and you might look a bit of a git. However, standing on stage a desperatley trying to get the E string just right also makes you look like a fool.

When talking to partners about the need for more instruments, however, ALWAYS explain that a different tuning requires a new piece of equipment (and bag, and plectrum, and strings, and strap....)