Most of the time you can just say something like "It should be unlawful to do X in situation Y because it undermines the ability of those involved to make predictably good decisions."
Rights are useful in the same way that asserting "pi = 3" is useful: it's simply better than nothing. It is hardly some kind of ideal, despite how easy life would be if it were true.
ISIS will say everyone has the right to have a slave. The UN says everyone has the right to paid vacations (yes, it really does say that).
Who decides which rights should be accepted?
There is no one who can do so. Rights are arbitrary and reflect the culture of those who defined them.
You either decide to accept them or you don't. i.e. it's pretty much identical to "rational argument".
He has tried to define a difference between them that doesn't actually exist.
This is a good thing because it provides a written moral standard and a legal basis for further argument - rather like a constitution.
Relying on 'rational argument' doesn't work as well because most 'rational argument' in politics, law, and economics is persuasion through sophistry, custom, tribalism, rhetoric, appeal to self-interest, and/or the application or threat of force.
None of these have much to do with rationality in the scientific sense of being able to build useful models of reality from data.
tl dr; Defining rights makes spurious political plays more difficult.