How should long-term investors think about flexibility versus full investment.?

In long-term portfolio construction, staying fully invested is often presented as the optimal default. Historical data and opportunity cost arguments generally support this view, especially over multi-decade horizons. Cash and other low-risk assets tend to act as a drag on nominal returns when markets trend upward. At the same time, some portfolios deliberately include flexibility through modest allocations to cash or short-duration assets. This flexibility can allow for rebalancing during drawdowns, reduce forced selling, and provide optionality during periods of market stress. While this may lower expected returns, it can alter portfolio behavior across different market paths. From a structural perspective, how do you evaluate this tradeoff? Do you view flexibility primarily as a form of risk management, or as an inefficiency that long-term investors should minimize? How do you incorporate this decision into an asset allocation framework intended to hold across full market cycles.?