I like the principle of the Prime Directive, partly because without it, you have no end of situations Starfleet could potentially get involved in.
It essentially asks, "what is humanity's role in the universe?"
Or, to what extent should humanity (or at least, Starfleet as a quasi-military branch of the UFP) be responsible for other races and what should be left to chance/fate/design/whatever-you-call-it?
Personally, I do not like the idea of humanity assuming responsibility for junior races development, as this results in unpredictable moral obligations and practical consequences. Should Starfleet have to constantly have to deal with this, potentially creating economic impacts on the UFP?
(yeah, yeah, they allegedly don't use money, but clearly they still have resource limitations: despite fusion & transporter tech, it is still possible to fight a war of attrition against them viz. the Dominion War.)
The PD is a (by and large) ethical, practical & common-sense way of restricting what situations Starfleet officers - as a proxy of the UFP government - assume such potentially unending responsibilities. Of course, there are situations where individual Captains can breach it at their discretion. They then have to later justify it to Starfleet/UFP and such a breach is noted as part of their personal record (I'm sure several eps across the various series either imply or outright say that).
I think Picard handled the situation in Homeward appropriately, given the circumstances he was placed in.