As I’ve blathered on about elsewhere, the intention of the Prime Directive is to prevent Starfleet from playing god with other civilisations.
That has plenty of benefit for Starfleet, in that it maintains their core principle of being peaceful explorers — there’s a very fine line between discovering what’s out there, and destroying what’s out there by interfering with it. The Prime Directive is there to keep Starfleet on what they believe is the right side of that line.
As such, it’s not ineffectual, because its goal isn’t for every civilisation in the universe to achieve warp travel without interference from alien civilisations. Its goal is to stop Starfleet from interfering with other civilisations, because a Starfleet that did that wouldn’t be Starfleet any more. They could end up like Q (toying with civilisations for fun), or the Borg (forcibly improving the quality of life for whatever civilisations they can reach).
With regards to what the Romulans and Ferengi might do — sure, they can go and manipulate the heck out of whatever civilisation they want. But practically, they’re unlikely to be able to reach all that many civilisations, especially those that the Federation might encounter, because space is really big.