LTFT is a long term average to keep things on par. STFT is quick instant changes which are always going to vary whether tune is perfect or not. The pcm wants to know that moving negative will lean it out and going positive will add fuel so it will constantly check this until it is satisfied. LTFT keeps everything close over time and isnt going to account for a quick change which is good. The pcm also relies on the LTFT numbers for other factors such as a referrence for PE fueling, if it is + or - for a long time then so is PE as it reverts back to this. If the car has a vacuum leak or other problem causing things to be off and you go WOT then the car's fueling will be off and possibly cause damage. Tuning with just STFT would be near impossible since it moves around so much whether the the tune is on the money or not. If it is WAY off to start with then it will help get you on the right path sooner, but once the trims are close to zero it is always going to hover around a little bit because there are too many varying factors such as temp, humidity, fuel variations, etc and the pcm always wants to see 02 crosscounts below and above 450mv so will do what it has to so it can constantly make that happen.
