to be an "open currency" where the supply increases as user-base/uasage increases is important.
Who says this is important? As more users join in, value will probably go up per bitcoin and people might hold fewer nominal bitcoins, but I don't think that makes much of a difference.
Any attempt to fix the value of a bitcoin in terms of price is going to fail, and is not necessary. Keeping the supply fixed is the fairest way to go about it and let the market decide the value.