I agree with your point entirely, but my point is that most people don't just sock money away for decades if it isn't specifically retirement money. Usually it is saved for some medium-term goal, which stocks could potentially spoil. Aside from that risk, why even deal with the psychological effect of that uncertainty for a paltry gain?
Most people can't come up with $2k in case of an emergency. Most people are living paycheck to paycheck. Don't be most people.
There is a compromise between living for the moment and living for the future. If you are saving for a house then you can move those investments to a less risky investment option but still a better return than bonds or interest. Keep your medium-term investments separate from your retirement account. I have short term, long-term non retirement, and long-term retirement accounts.
The gain isn't paltry. The difference between a 2% rate of return and a 5% rate of return for $5k initial + $50/month over 30 years is around $22k. If you do $200/month it's $59k.
