jtimon,
if you produce something that costs 8 EUR, and then receive payment in the nominal value of 9 EUR (even if it's actually settled with a complementary currency), then you still book 1 EUR profit.
If you however receive a payment that is at the time of payment worth 9 EUR, but has a fluctuating exchange rate, you still need to pay most of your bills (e.g. salaries, rent, raw materials, electricity) denominated in EUR. You might have debt denominated in EUR. If the currency you receive is liquid (e.g. an existing foreign currency), you'd probably sell it right away, or at least take a position on the futures market to hedge against exchange rate risk. If it is not liquid and you cannot use it to pay for your bills, you've introduced a risk element into your business plan, because trades that would be profitable with a fixed exchange rate might become unprofitable.
Yes. But maybe you're using 1970usd (or 2002eur or whatever) precisely to hedge against inflation. But, yes, you would need to adapt your business plan to that (that may include not accepting the full payment on the complementary currency, just a percentage).
Bit-Pay (and in the meantime probably other merchant systems too) use the relatively high liquidity of the Bitcoin exchange market to compensate for the exchange rate risk. But they only have a market because using Bitcoin even when there is forex involved can decrease total trasnaction costs. If it wasn't for this, noone would use them.
This has to do more with the price volatility of bitcoin (due to its relatively small economy), but I agree.
Businessmen cannot offset this risk by increasing their prices, as that makes them less competitive. I agree that if you only accept partial payments in the compementary currency, you reduce the risk, but I don't this this is sufficient. But I admit that there could be an empirical component to this.
In Volos (Greece), there's a LETS currency denominated in euros. Most business only accept a proportion of the price in the complementary currency.
That's not only enough to cover their costs. If they weren't accepting it, they would sell less. There's a little documentary on this, but I can't find it right now. Everybody's willing to spend their "Volos", but people are more hesitant to spend their euros. Although euros are a liability of the several central banks in the eurozone and the ECB, they effectively act as "scarce/anoynmous/cash money". In the other hand mutual credit is "abundant money", as it can be created when is needed and destroyed later. There's no point in hoarding abundant money and you can't ask interest for it (people can just create it instead of borrowing it from you).
This is, in my opinion, not only the reason why complementary currencies are pegged to local (rather than foreign) fiat money, but also why the peg sticks. People do this to minimise their exchange rate risk. The peg creates an equilibrium due to the network effect of the local fiat money.
In order to avoid this, you need to diminish the network effect of the local fiat money (e.g. increasing autarky). Then you might be able to shift the equilibrium. The strength of the network effect is an empirical factor.
I think this is an important factor but I would say simplicity in pricing is also important. As you say, self-sufficiency (autarky) and more localized economies care less about those risks and are more suitable for using also an alternative unit of account such as hours or the "national" currency inflation adjusted.