Worker pay goes up with unionization and collective bargaining without needing a tight (usually temporary) labor market. Otherwise, we end up in a highly cyclical economic condition where employment conditions are tolerable during tight labor markets, but are downright terrible when there is a surplus of labor (note only recently, with unemployment near 3.9%, the lowest unemployment rate since 2000, that employers are considering usually undesirable candidates [criminal records, no college degree, etc]).
Why is this undesirable? Simple. Lower and middle class workers drive the economy with their consumption. If they don't have stability, they don't spend. If they don't spend, the velocity of money through the economy decreases. You want workers to feel comfortable spending, and the only way to do that is by giving them stability along with a fair wage.