is there any proof or write up that states u should always put
ur transmission into neutral so as not to overwork the engine?
there are merits to both practices i presume.
constant engaging and disengaging between "D" and "N" causes the
torque convertor additional stress because of the fact that it runs
on a fluid coupling effect. ie; the harder u step on the throttle,
the more fluid gets forced into it and the more forward motion it
generates.
at idle, the amount of thrust generated by the torque convertor as
a result of the engine's speed is minimal but optimal for move off.
if u engage "N" at this point, the pressure build up in the torque
convertor is relieved. when its time to move off, and u reengage
"D", the pressure will take time to build up to turn the convertor
again to generate the same forward force.
now, u dun need a phd to understand that this constant changes in
pressure using the fluid coupling effect and the effective
fluctuations in the rotational speeds of the mechanical parts will
result in pre mature wear and tear.
likewise, one can argue that the engine will be "stressed" by
constantly working to provide this pressure build up to keep the
torque convertor under such conditions. but IMO, if its anything
thats the weaker link here, it's gotta be the automatic
gearbox.
i've seen more breakdowns in auto gearboxes but hardly in any of
the engines running them. which goes to show how well informed some
drivers of automatic transmission are in this aspect of care for
their gearboxes.
what is "N" for exactly? i can only think of one logical question
to ask in answer to that. try pushing ur car in "P" when it breaks
down. 
