I see how both scenarios are possible. In either case, when the MAF goes out, the strategy will go into FMEM mode and substitute values for the failed MAF. The difference between the two senarios comes into play due to the adaptive learning algorithm for the MAF.
You see, values are stored in KAM (Keep Alive Memory) for the MAF and this process is called adaptive learning. If your MAF adapted on a really dirty, poorly functioning MAF, the default value substituted for the MAF, may not play all that well. Thus the vehicle will idle poorly or even stall. If the values in KAM are based on a clean MAF then you may not have much of a problem.
The values in KAM are the reasons it is advised to reset the computer after one cleans the MAF. Force it to relearn and store values in KAM using a clean MAF.
