An aspect to the multiple currencies idea that I haven't seen discussed relates to the tendency of markets to squeeze out redundant inventory. The examples I was noodling on were Beta vs VHS and IBM vs The World in the early days of personal computers. Both markets moved toward de-facto monopoly of a single design (although manufactured by multiple vendors).
This was a wonderful thing for third-party people who had to incorporate aspects of these goods into their processes (video duplication & rental, software production and sales, etc.)
With currency you have third parties like vending machine makers, retailers who don't want two- or three-story cash drawers at checkouts, etc., all wanting a single form factor and design. Frequent currency innovations also not welcome here.
Is there really competition in useful areas that bring customer benefits... if retailer wants to give me change, and has only X-brand currency, but I prefer Y-brand currency, will I really turn down X, assuming both equally negotiable? Similar to Coke/Pepsi: most people have preference, but will accept other if only one available. Competition has heavy distribution game component, starts to swamp price/quality component. Leads to high marketing expense and high marketing as percentage of cost. Retailers hold high cards here, can bleed currency suppliers, possibly extracting most profit from 'currency supply business'.
Commodity product with distributors in command. Ugly business for manufacturers.
This problem of multiple notes is exactly the hell the Euro faces in January. There is a pretty good special section in last week's Economist about the sheer difficulty of moving coins, bills, managing multiple tills, etc. The more these governments screw up, the longer they take to exchange the currency, the more old (and subsequently valueless) cash the people will be stuck with. The transfer is supposedly going to take only 30 days. Can you smell the scale of this rip off? Note that your VISA card doesn't have that problem. My first thought is that the problem isn't the currency, it's the use of physical exchange media consisting of coins and paper.
I don't think multiple exchange media will work very well without EFT (electronic funds transfer) but this looks like very fertile ground for some clever inventor who could come up with something better than paper and coin that could do the trick. (That's the cogitating part.) "Coming up with something better" could be tough, but I don't see it as at all insurmountable. It took a long time for people to accept paper (and some are still bitching about it as we see here). The key is that it is useful, decrementable, secure, private, easy to use, and tamperproof without need for an instrument. In that respect, we might well see multiple exchange media used in specific sectors or locations with the currency exchanged at the door, much as we see tickets and tokens at amusement parks.
It beats stuffing penny parking meters with gold coins.