Technically, there's two kinds of rationality in play here. If we only take into account the perspective of the single music consumer on the shortish term, then certainly one ought not to buy an album when one already has the music. That money could be spent in other ways which would add to the happiness one has already secured by getting that music without paying for it.
If we widen our gaze instead and take into account *the long term interests of both music producers and music consumers*, then we really ought to pay for the music we listen to. Because otherwise musicians would be working for free, and this is isn't right. It also isn't sustainable long term. Even if we only take into consideration the perspective of the single music consumers, what would bring them short term happiness, may bring them longer term unhappiness. Why? Because it may well be that *in time* those consumers will be left with *less music* to listen to, whether they buy it or download it illegally. (The music business might become less and less attractive to young artists if one doesn't get paid for making music.)
Similar considerations apply, for instance, to environmental matters. It is in our short term interest to produce plastic, burn fossil fuels etc. And it is perhaps in my interest to drive from x to y instead of using public transport. But, once we widen our gaze a little, we can see that these short term interests are very likely to be massively outweighted by unsustainable climate change costs.
So, in a nutshell, I think both parties in the foregoing discussion were right, because they were to some extent talking past each other (they had in mind a different conception of 'interest/rational'). A single person's interest over a certain time interval t (what may be rational for one to do at t) may not coincide with the interests of larger groups over a larger time interval t'.
Of course, my own thought is that, when our actions can affect society as a whole, in deciding what we ought to do, we should take into consideration the interests of larger groups over larger time intervals.
Concerning the music industry: the million dollar question is, to be sure, how to make people pay when everything can be pirated so easily. U2 are precisely trying to answer this question, even though it is questionable whether the SOI stunt really helps the general cause in this respect. SOE might be different.
My two cents
