Off the top of my head I think that calling it the "right to revolution" is something of a misnomer. I would subscribe to some kind of Hegel-influenced model of rights, where rights exist by virtue of their being demanded and enforced. In a typical modern society, it is largely the role of the state to enforce rights, via mechanisms of the judiciary and penal system. So to talk of the right to revolution - an action which has as its essence the destruction of the body that is typically responsible for the enforcement of rights - is a bit misleading. Someone who believes rights have some kind of divine or non-anthropocentric origin might disagree.
But having a right to do something and it being morally permissible or even morally desirable are not the same. If you believe that what is morally right or wrong is largely a function of the kinds of social and intellectual relations that characterise a given society or community, then you might say that if enough people feel oppressed to the point where they rebel, then the revolution is de facto morally justified. But, at the same time, this assumes that what the majority think is best is what is actually best. And even if that were the case, it's not obvious that revolutions, even so-called 'popular' ones, always have the support of a majority of people in a country. In Iran in 1979 or Ukraine in 2004-5, even though hundreds of thousands or millions of people filled the streets, many times more than that stayed at home. Obviously not everyone has the mental or physical strength to protest, but even taking that into account, there are always many people in favour of the status quo.
And I think it's worth considering that how moral a revolution is will largely be judged historically by the actions of the regime that came to power after the revolution, rather than the regime that was ousted. The cruelties and massive inequalities of Tsarist Russia do little to cast the Gulags or the Great Purge in a better light. What you get after a revolution will always be different, but different is not necessarily better.
At the end of the day I think that popular revolution is one of the purest types of democracy in action. Whether it is morally justified or not, and irrespective of what the outcome is, it is the voice of the people made concrete in radical structural reform. For good or for bad, the public gets what it wants, and then lives with the consequences. The very fact that events are dictated from the bottom up, rather than from the top down, counts for something, I would say.
Off the top of my head.
