Anytime a series lays down rules and breaks them continually instead of as an exception is usually when writers back themselves into corners, an example of bad writing. Its no coincidence some of Trek's worst episodes were prime directive breakers.
I think some of the series best episodes involved the Prime Directive. Errand of Mercy
would be considered a 'Prime Directive breaker' and it's one of the best episodes of any Trek series. A Private Little War
and Too Short a Season
are both very powerful episodes about the Prime Directive.