It's not called dilution, it's called competition, and it's a healthy part of a market that's not regulated by the state.
It's only dilution if one clings to one specific currency with religious fervour and refuse to diversify, and it happens to lose market share to a different currency. Then one's personal portfolio will be 'diluted.'
You talk of bitcoin as if it were an investment commodity (and many others in this forum talk about it and use it that way). In that case, yes, there's no problem. Trade whatever you want and see what happens. It's a trading experiment.
But bitcoin is marketed as a currency and a currency needs to have some universality to be successful.
How would transactions for tangible goods and services happen with mutliple digital currencies? Would merchants accept a number of them, like they do now with credit cards, or would there be some automatic way to exchange a digital currency for another on the fly?
I also think of the bitcoin community as a social network. There are a few social networks, but the ones at the top have established themselves and it's difficult to dethrone them because of the network effect. On the other hand, there are things like telephone operators, which have found a way to interoperate seamlessly by using common protocols. So I can use the telco of my choice and you can use the telco of your choice, and I can still call you and talk with you on the phone.
