S&P500 fills the US large cap asset class in one's portfolio. It can generally be had at a low expense ratiio. For example VIIIX has an expense ratio of about 0.03%, SPY has e.r. of about 0.09%.
Many folks would recommend using a total US stock market index fund instead of just a US large cap fund. Such a fund would obviously fill the US large cap asset class as well with the additional benefit of giving you mid and small class.
There are other US large cap index funds that could fill this asset class.
If you don't believe in asset allocation and asset classes, then I guess this discussion could be moot.