More to the point, strict adherence to all the rules leads to mechanical production. And this is boring. For example, one hopes the singer is always on pitch, but singing a fraction sharp lends brilliance and immediacy; a bit under the note mellows it. The rule is: sing on pitch; interpretation and expression arise in part from bending that rule. Rules on using chest voice, selecting repertoire and so on are similar in that bending them has both cost and payoff. Or, similarly, the use of rubato. Without it, again the singing sounds mechanical. The slight "borrowing from Peter to pay Paul" as Marylin Horne put it puts elasticity in the overall tempo. But as Mike put it with pitch, it's tricky. There is an almost invisible line to using rubato and pulling the line out of shape.