Still not sure it's of much use to me. I travel a few times a week from
Zone
3 into central London, an occasional bus ride, and a few Southern trains
between Balham, Victoria, Croydon etc., on which I understand you can't
use
Oyster.

I find it is worthwhile just for the convenience of not having to use the
ticket machines for the trams or for buses where you need a ticket for
boarding, or having to mess around paying the driver. Saving a few bob in
doing so is a bonus.

Would there be any point in loading a one day Travelcard (say zones 1-4)
onto an Oyster card on a day when I need it, or might I just as well carry
on buying paper tickets?

You can't actually load a one day travelcard onto an Oyster. Although I'd
find it useful myself I can see why they never bothered with it as it is
just as easy to hand you a paper ticket.

G.