In simple terms, a domestic investor's local-currency-denominated return in a foreign security (or a portfolio of them) is equal to the foreign security's (or portfolio's) return plus the foreign currency return, plus the product of the foreign security return and the foreign currency return. The last part of this equation accounts for the interplay between the two, and as it is the product of these two figures, its contribution to the overall return will grow as either the foreign asset return or the foreign security return grows larger.
It is useful to look at historical data to frame the effects of currency hedging on investment performance (for U.S. investors in this case). There are two key elements to consider when assessing the effects of currencies on equity portfolios: their contribution to return (as covered above) and their contribution to risk.
The best answer to the question of whether it makes sense to hedge the currency exposure of an international-stock portfolio is this: It depends. By hedging foreign-currency exposure, investors can mitigate a source of risk--but at the expense of a potential source of return. The trade-off between the two is important, and investors' decisions will depend on a variety of factors, including but not limited to their return requirements, risk tolerance, investment horizon, and the costs associated with hedging currency exposure.