My reasons for doing as much as I can on my bike:
1. I like doing the work, it's relaxing, even if takes me practically forever to get simple things done.
2. I feel more in tune with my bike when riding, when I have done the wrenching.
3. My mistakes are learning tools (after I stop cursing myself). for example, I am paying the price for my chain oil/chain choices. New chain and cassette this weekend!
4. I get to buy more tools. I buy cheaper when I can, but get better when I can.