By making the suggestion to have the "work" being done be more "useful", you have essentially shown that you haven't taken the time to read and learn about *what* is actually being done by the mining rigs.

The problem is that the "work" is actually calculating the hashes in order to find a block, above a certain difficulty, that can be added to the block chain.  In order to plug into another parallel computing project, you'd have to find a way to make it still add to the Bitcoin block chain in order to continue to verify the transactions in those blocks. 

That would most likely require the parallel computing project to work with the Bitcoin project and divvy up the work for the project so that we solve a block for them, they solve a block for us. 

Unfortunately, they're probably not going to cooperate, and it would mess with the Bitcoin economy, because now you're artificially changing the time it takes to find blocks.  Not to mention all of the work it will take to make the protocols and "work units" interoperable.

It's just not practical.  Mining takes resources, because it's supposed to take resources.  Mining isn't supposed to be "free".