To the operators' point of view, as long as the bus is operating
well, meet their needs, and also value for money, they don't really
bother whether what body is it. And most commuters don't care what
bus is that as long as it is punctual, efficient and clean, and
most importantly doesn't raise bus fares. The recent purchase of
Citaros by SBST and SMRT is probably just a bonus for
commuters.
However in future to increase bus fares to cover the costs to
buy more of these buses in future, I think bus operators here need
to think twice, based on lifespans, usages and costs of
returns.
Anyway, I agree that Gemilang is not a suitable brand for the
Scania KUBs and Mercedes OCs. SBST should have used Wrightbus or
MCV or even East Lancs to body the KUBs. Even Soon Chow may be
better than Gemilang.
Actually, its pretty true because most of the maintaience wont
be on the body, but still it probably wont give a good impression
on the pax and then it will lead to some other complain. Just like
if the bus u are taking is slow, u will find some other complains
about it and so on so forth. Thats what i feel. 
But still the bodywork designs do still make a slight impact on
efficiency, for eg. placement of doors, chairs, grab poles,
staircases and different door designs(plug, sliding, leaf, etc)
will still impact on some degree to the turn around of pax at bus
stops. This is probably why sbst opted to get wright to redesign
the lower decks to make the B9's lower deck stand more pax compared
to the orignal designs used by kmb, dublinbus, etc.
However i still do hope that bus fares dont get raised due to
better vehicles, its their responsiblity to serve the public while
still keep having improvements and accessiblity to users. kmb could
do it, hope they can manage to do it too.
Last of all, lifespan, usages, cost of returns are important as
per what u have said, but there stands a more important factor,
which is fuel consumption. As fuel prices continue going north, it
will probably cause a impact to fares sooner or later when the fuel
reserves runs out and a steep fare raise will then be immenent.