The bitcoin network isn't just computing numbers for the sake of computing numbers. Proof-of-work is integral to the "decentralized network of trust" that is bitcoin.
Also, even if bitcoin became the dominant global currency, I highly doubt that it would take up a noticeable portion of the world's computing resources.
That's the really interesting bit about Bitcoin - it's an attempt to create a system that will enable trust in Internet-scale groups.
Currency is obviously a prime candidate for "trust", but I also think Namecoin is tremendously interesting. Next up, PKI certs? Eliminating hard-to-scale single points of trust (whether the DNS roots, VeriSign or governments) is hardly "doing nothing".
I haven't taken the time to fully understand it, but I gather that there are proposed ways to trim the block chain's history. For clients that don't need a long history, I imagine they could just download a snapshot for, say, Jan. 1, 2013, that simply listed all the bitcoins and their owners at the time, rather than all the transactions that happened before that point.
Also, even if bitcoin became the dominant global currency, I highly doubt that it would take up a noticeable portion of the world's computing resources.