Never use the same strat twice? So u dont use metal vs toss? U dont use mutas vs zerg? U dont use reavers vs toss? Thousands of players discovered that those strategies worked better than anything else, but u have to try a broodling rush vs protoss because u already tried to make zerglings in early game?
There is a "right" strategy for every MU. Of course, u have to vary ur builds and units to counter what the enemy is doing or catch them off guard (for example M&M rush vs toss or wraith rush vs zerg), but there still is no better strategy than M&M in TvZ, reaver/sair on island maps or drops/tank + BC in TvT.