It does all depend on our respective definitions of decent!

If it's a zone 2 Travelcard then I've found a number of rail journeys
that either I or my associates would use in south London. Clapham to
Camberwell by bus can be a bit tortuously slow during busy times
(including weekend afternoons), but Clapham High Street to Denmark Hill
is a breezy 5 minutes (though it only breezes every half hour).

Also Elephant & Castle/ Denmark Hill/ Peckham Rye to Lewisham, with the
possibility of interchange onto the DLR up into the Isle of Dogs.

Plus if you lived near the DLR it'd be very useful to have a zone 2 (or
even zone 3) Travelcard.

And lots of other single zone rail/ tube journeys that could be useful
to some are apparent - East Putney up to Earls Court or Hammersmith on
the District, Clapham Junction to Vauxhall by rail (a frequent service)
then onto buses if needed, Beckenham into Bromley South, Sutton to
Croydon, New Cross (Gate) to Whitechapel - with a change at Canada
Water to get to Canary Wharf or North Greenwich, Hackney Central to
Highbury/ Camden/ Kentish Town, Clapham (South/ Common/ North) to
Elephant and Castle. Have I bored you all to death yet?!

My point is that if a Bus Pass is purchased partly on account of it's
economy value, many passengers might as well splash out a bit extra if
there's the possibility they might complete such an orbital journey, or
perhaps if they're travelling into town (zone 1) by bus they could jump
on the tube / train to a major bus interchange (i.e. Vauxhall or
Elephant and Castle) which may speed the journey up. Or, as Mr Arquati
points out, when using Oyster pre-pay it would make the occasional tube
journey that bit cheaper as you're only paying for a zone 1 journey,
rather than zone 1 & 2.

Apologies for labouring the point!