One good reason to index is that the professional fund managers often can't beat the market, or even keep from loosing money. My 403b (AXA/Equitable) lost a lot of money the past several years. The only thing worse than loosing money through your own mistakes is loosing it through someone else's mistakes. Looking through the investments available to me through my 403b I see a lot of the funds are still loosing YTD, and some that are "up" are making a very small returns. (As an aside, I've signed up for online management so I can choose investments now rather than just going with what the "advisor" um, advises. This is not a good thing. A dummy like me has no business picking his own investments, but when the "pros" are loosing me money, what's the alternative?)
I recently opened a Vanguard Roth and have bought some index funds. They will go up and down year to year but in the "long run" (10 years for me) they ought to perform reasonably close to their benchmarks. If I had a longer time line I could be even more certain that the index would perform very close to the benchmark. This, as far as I can see, is as close as one can get to "locking in" a return over the long haul. The market -or a selected benchmark sub-portion - historically returns X %, so over the long run, the index will return X %. To do well, long term, people don't need to beat the market, they just need to keep pace with the market and compound. Indexing does that.
That said, since I have no choice but to choose my own funds for my 403b, I'll probably be willing to risk 2-3% of my investment funds on picking my own stocks. I see this as forcing myself to learn more about the market. I'm willing to take the small loss if I make bad decisions. But over all I'm much better off keeping 97% of what I think of as my "Vanguard money" in index funds.