> a home water filter might also solve that particular problem

Yes. I don't see why it's an issue. A lot of houses have water softeners because hard water destroys appliances. An RO filter for the kitchen isn't a lot more expensive than that.

(And I've also lived in developing countries where the water isn't always safe to drink. Boiling or filtering worked fine).

Makes some sense, still, especially for renters, if they don't want to have to move the system and have a big (yes, it's big to some people) up-front expense. If it's just for drinking water, it looks like you can get over 2500 person-days of drinking water in the form of CostCo 16.9oz 40-bottle packs ($2.95 for 40 bottles) for the same price as a cheapish RO system (~$200—you can buy about 333.3 gallons of that Costco water for that). About half a liter of plain water intake per person per day on average, in the US, from what I can tell. Even if you double that to account for tea and coffee, that's about two years' drinking water for a couple.

Considerations: that's not even the cheapest filtered water one can buy, and the RO system has consumable filters (= more expense—frankly, I wouldn't be surprised if the bottled water is straight-up cheaper, overall), maintenance, and installation to worry about.

I don't think buying bottled water regularly is necessarily irrational. FWIW, we do have an under-sink filter on our kitchen sink rather than buying bottled water, though I'm not convinced it's justifiable based on cost—we don't live in so-bad-you-can-smell-the-water land anymore, but the filtered stuff still tastes a lot better, and it's nice not to have to remember to stock up on water at the store, though we're so bad about maintaining the filter that I think if we did it right the extra time and more-stuff-to-remember would make buying drinking water more convenient after all)