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The key issue here is trading costs. Rebalancing a portfolio to consistently maintain an asset class’s target portfolio weight requires more or less constant trading and the inability for the manager to time the trades. Both of these factors mean that rebalancing to a target portfolio weight results in higher trading costs. Note that the higher trading costs could increase the tracking error of the portfolio versus one that has allowable trading bands. Also, even though trading costs would be reduced by having an allowable range for each asset class, the manager would still need to constantly monitor the portfolio to see if the asset classes are still within the allowable bands, so there is no discernable difference in the amount of monitoring on the part of the portfolio manager.