Different uses.
Sharpe is simply a measure of an assets expected return relative to risk, instead of (some) people who just measure performance on absolute return regardless of measuring how much risk was taken on.
To try and make the point, I would avoid calculating these things, because asset classes will experience a very different volatility profile in the next 2 decades than in the past two. US Treasuries, as an example, have been extremely volatile over the past few years.
If your real goal is to minimize volatility, then by all means go ahead with your approach. Your real goal should be to continue adding asset classes with expected real returns (i.e. outperforming a risk free return and/or inflation) within your portfolio which are relatively uncorrelated.
If you're into calculating stuff, I'd suggest you try a macro review of asset class volatility (i.e. US large cap, Intl large cap, Intl munis, US munis, REITs, p2p, etc) and see if you can come up with a portfolio with weights that you'd be comfortable with the total expected return, and that also has a relatively low Beta to the S&P (or Dow/Nasdaq/whatever).