One of the points which used to be made was that high flying aircraft are in the stratosphere, where weather systems don't reach.
Consequently the deposits from aircraft would remain and accumulate in this layer.
The observations after 911 seem to indicate the stratosphere somehow became clearer in four short days, so it would appear that aircraft deposits do infact get reabsorbed in the troposphere, and are therefore not too damaging long term.
No doubt this is an oversimplified or just plain wrong interpretation, so I await other interpretation of the observations.