As i understand, there are two main reasons for using dilute perceptol (specifically).
1. to slightly reduce the effect of the grain disolving ingredient, thereby increasing accutance (although, this doesnt seem to leave a noticably higher grain in the negative, just better edges); and,
2. to effect a controlled exhuastion of the developer while it is working which serves to compensate for high subject brightness ranges, although clearly not to the extent of two bath or water bath development. For it to work effectively, agitation and stand times must be well ballanced - i use slow but constant agitation for 30 secs, then one inversion of the tank every 30 seconds and find this is sufficient for two 120 rolls in 600ml of 1:3 perceptol, then as it stands, the shadow (thinnest) areas of the neg continue to develop as the highlight (densest) areas quickly run out of steam.
I have not noticed any substantial overall thinning of the negative using this method and find it gives me fine negatives for scanning and/ or printing.