I guess perfect was too strong a word. I guess what I ment is the assumption is that what they are teaching is what worked on the battlefields of ancient japan. For this to be true, they would indeed have to have perfect transmission from teacher to student. You and I know this can not be the case, so how do we prevent the degradation without sparing?
I liked what you spoke of with regard to graduated resistance. This occures in Aikidojo to varying degrees, so i guess the question becomes a matter of to what degree it progresses and up to what point. The only way we can prevent degradation is through diligence and sincerity of training. Sparring has existed in plenty of MA which have degraded over time, so i don't think sparring alone makes the difference, though I do agree it is one way of testing the hypotheses.