Originally there was a strong motivation to have mining enabled, as the cpu requirements to generate a block (and thus win the 50BTC+fees bounty) were minimal. These days, there's SO many computers out there mining (and using custom mining tools that use the GPU instead of CPU which is far more efficient) that on my 3-year-old PC the average time between block generations using the native client is some 1500 days! Even with a GPU miner it worked out to be 1 block every 120 days.

If you've got a reasonably new PC, run a GPU miner and connect to a mining pool, you may find that you can mine a few hundred coins per year in the background and that's certainly worth it to enough people that some of the mining pools out there are doing over 100Ghps. By comparison my GPU was doing 27mhps, and my cpu was doing 4!