Cryptocurrencies Could Crunch Meaningful Code Instead Of Burning Electricity

I can see the immediate appeal of the idea.

However, Bitcoin is obviously still going to be the most popular one for a long time, simply because it was the 'first-to-market'.

Without enough adopters, interest for secondary cryptocurrencies might naturally fade off, not to mention it might not be profitable for any exchange to keep trading them.

From a quick look at https://btc-e.com/, it seems most of the value is placed on Bitcoins and Litecoins atm, both which crunch 'meaningless' tasks as @leashless mentioned before me.

Another important fact to add is that there is an upper bound for the total amount that can be mined for these currencies.

This further provides them with value, ie.: they are somewhat scarce resources, not unlike gold or other precious metals.

As for prime numbers/Primecoin, I haven't looked at all the details, but I believe there is an infinity of Cunningham chains?

There's a good bit of (financial) theory that suggests proof-of-work systems have to be meaningless to work.

Successful practice could overturn that opinion, but it's well founded. Nick Szabo's yer man here.