Kang:
fuel injector cleaners that contains stoddard solvents or similar will smooth out the idling due to the stratification of the fuel during air mixing (atomisation) when fuel injectors spray. Depending on the charge temperature, the swirl of the air during intake stage and also many other factors, combined, as the F+A charge "tumbles" and mixes, some heavier aliphatic and aromatic hydrocarbon compounds may "statify" the charge, altering the flame propagation pattern during the ignition stage.
Some engine designs (combustion chamber and valving positioning, number of valves, intake air charge and spray pattern, compression ratio, "tumbling" effect of the F+A charge during intake and compression stage and more...) may benefit due to the slower propagation of the flame propagation pattern after the introduction of some sort of aliphatic/aromatic hydrocarbon compounds, leading to smoothing out of idle roughness, a slightly better acceleration due to "quenching" effect.
As a car ages and mileage accumulates, there shall always be some gasoline related deposits accumulation in the upper combustion chamber wall, piston top, top compression rings and ring lands, exhaust valves.
Intake valve deposits, on the other hand, comprised of mainly carbon deposits caused by (a) motor oil fumes introduced by PCV (closed crankcase air(fume) recirculation) and (b) coking to the fuel remains in the combustion chamber the moment the engine shut off. As time goes by, this "coking" of carbon will have a detrimental effect on the fuel+air charge pattern during intake stage, the heated (sometimes glowing hot) carbon will attribute to the tendency to pre-ignite of the charge, causing serious drivability problems.
So, unless you can afford to tear down the engine and do some combustion chamber cleaning and intake /exhaust valve cleaning to rid of all sorts of deposits, otherwise, you shall experience some form of mild drivability issues such as rougher than when new idling, sometimes even acceleration problems.
Nice cars with full computer control and compensation will have the ability to adjust to these changes with knock sensing, etc. to keep the situation under control. As a result: luxury cars seems to fair much better than some lower-priced counterparts (with less computer management controls).
As for pintle versus pintle-less injectors, yes, you are right. Pintleless injectors are introduced to specially oversome certain kinds of coking conditions (fuel remains when the engine shuts off will cause carbon formation on the injector tip(pintle-type), and as mileage accumulates and that deposits becomes bigger, it will alter your spray pattern causing driveability issues). The good news is that most German and Japanese made cars they delibrately positioned their fuel injectors to overcome the possibility of deposits due to coking, as a result: unless you are running some extremely crappy fuels with insufficient deposit controls and coking controls and/or oil burning issues (such as excessive valve stem to guide wear causing motor oil burning, PCV valve causing excessive introduction of motor oil on the intake manifold side, etc.) otherwise, most Japanese and German cars can live a fairly long life w/o the call for drasatic fuel injection cleaning.
Also: don't disregard the importance of replacing your fuel filter on EFI cars regularly. The life of your injectors are depending on it.