Well, I respectfully disagree about excluding dividends. Let's look at this example: Two companies were both priced $100 a share 4 years ago, and both are currently priced at $200 a share. Company A paid a 2.5% dividend while company B did not pay any dividend. Clearly company A had a better return than company B. Because of this, it does make sense to incorporate dividends into investment returns.
That said, not only is the point regarding taxation spot on, it makes return nearly impossible to calculate due everyone's unique tax situation. Candidly, I don't have a holistic calculation in mind, but assuming dividends are reinvested and conservatively assuming a 35% tax rate on those dividends would give a reasonable ballpark figure.
Folks posting here tend to have longer time horizons than most, and those longer time horizons make reinvested dividends substantially more valuable than it does for short-term traders.
-Crip