I believe rebalancing may be beneficial in one specific case, where the expected returns of the two asset classes are about the same, while the correlation coefficient (covariance, I believe) is not 1, and where the allocation between the two classes is 50/50.  Maximum benefit occurs whenever correlation coefficient is -1, and volatilities are high.

For example, if you expect domestic stocks to return 10%, and you expect foreign stocks to also return 10%, the a 50/50 portfolio, rebalanced (I seem to recall that there is an optimum timer period, but don't quote me!), would be expected to ourperform the same portfolio without rebalancing.

I also think something like a 50/50 S&P500 fund/VWEHX might also qualify.

But certainly the traditional stock/bond portfolio, at any allocation, doesn't qualify.