Bendies will still be needed even with whatever kind
of 12.8m three door two staircase fancy DDs....
The problem with short-distance feeders or feeder-like services
is, people are not moving upstairs not because of DDs having not
enough staircases to go up/down nor that it takes a long time
to load/unload (most feeders have unidirectional loading anyway),
but because there's no point of going upstairs when you will alight
1 or 2 stops later.
Some SBST feeders works (or worked) well with DDs,
but because there's simply no option. Given two choices: going
upstairs so that you can board; versus not going upstairs and
having to wait for a long time (as some feeders had relatively poor
frequencies in the past), many would choose the first - which
only means that DDs are better than SDs. If SBST had bendies, I'm
sure they would deploy bendies instead of DDs on these slots.
(That should be the same reason why 72B operates with majority SD
fleet)
The 12.8m three door two staircase fancy DDs (referred to
as "12.8m DDs" below) that are often in discussions these days
helps the most, IMO, in situations when there is huge
boarding/alighting activities at the same stop, when
single-staircase DDs face problem because boarding pax need to wait
for others to alight first. Some examples that I immediately
think of are 52, 78, 154, 201 at Clementi, 222 at Bedok MRT,
and 72 at Hougang. 12.8m DDs are badly needed for these
services. However, this situation DOES NOT apply to most feeders as
their loading patterns are often unidirectional. so you don't see a
lot of pax both boarding and alighting at the same
stop. (Those saying services like this need
bendies just because of long boarding/alighting times, stop
trying.)
The only issue of using bendies for feeders (NOT trunks) is
space constraints, including the space required at bus stops /
interchange berths, in bus interchange parking lots, and in bus
depots. For the first issue, although it's true that feeders
usually ply areas without other services, they still need to
load/unload at bus interchanges and major bus stops, where you
often see a lot of buses waiting for alighting berths or waiting to
enter the bus stop. Bendies will make the situation worse. For the
other 2 issues (parking lots), although there can be changes to
existing infrastructure, it often result in reduced number of
lots.
In summary, my point of view: Use bendies wisely for feeders and
feeder-like services; 12.8m DDs for others. If no bendies,
add buses to achieve reasonable frequency first (not 291/293 kind),
then upgrade SD->DD.