Understanding and investment go together: The more you understand something, the more you should invest in it. On Friday, Bear Stearns owners thought their stock was worth $30/share; they were utterly wrong, it turned out.
In this sense, self-experimentation — research so cheap it can be done as a hobby — is a statement of complete ignorance. Because it is so cheap, you can test a hundred absurd ideas. If you use more expensive research methods, you cannot afford to test ideas you think are absurd. You must search a smaller solution space. If you are not correct about where the answer to your question will be, the region of possibilities that contains it, your research will fail to find it.
My self-experimentation about why I was waking up too early revealed that I was almost completely ignorant about what I was studying. Two of the causes I found — eating breakfast and not standing enough — were not on my list of possibilities when I started. The Shangri-La Diet is outside the range of weight-loss methods that obesity researchers consider reasonable; without self-experimentation, it would never be tested.