Short term ups and downs have no significance if your total portfolio is compatible with your tolerance for market volatility. Professional market timers claim to reduce the volatility in their customer's portfolio. However they must move large portions of that portfolio between equities and bonds or cash to effectively accomplish the stated purpose. Their decisions are based on various technical indicators, oftentimes mutually contradictory. Whether correct or not, making large changes is difficult for many investors psychologically.
Great discipline is required in following a particular market timing formula.
When losses occur, as they inevitably will, there is a strong tendency to abandon the formula. This often makes things worse than they otherwise would have been. There are few, if any, professional market timers with successful long term records. Consequently in order to promote their systems they must either quote short term results or present back-tested performance using historical market data. Performance based on back-testing is notoriously unreliable.