The iambic pentameter owes its pre-eminence in English poetry to its genius for variation. Good blank verse does not sound like a series of identically measured lines. It sounds like a series of subtle variations on the same theme.
Metrics are not a device for restraining the mad, any more than 'open form' or free verse is a prairie where a man can do all kinds of manly things in a state of wholesome unrestrictedness.