I agree re the inverted aircraft scenario. Then you have to kick out the rear windows.
But this seems very rare. The POH procedure for a rough landing is gear-up
, and from my reading of accident reports very few of those end up inverted.
I also don't see a special issue with ditching. A number of TBs have been ditched, successfully. One quite recently as it happens, and he was in the raft with just his feet wet (I know him).
If you decide only to have one door however, why not have it in the pilot side? It is not only Piper, also Mooney, Beech and others, so surely there is a good reason!
I suspect the reason is to enable passenger(s) to get in and out without the pilot having to do so as well. Historically, many years ago, these types would have been used for a lot of paying passenger transport ops. But that's only my guess - it goes back to at least the 1940s.