Very good question. What I believe is , during development , we check the time vs release profile upto a period of 120mins max for IR tablets. We compare it with RLD , also subjected to same condition as that of the test and try to understand how far or how close our test product is from the RLD by comparing the release profiles. So our main intention in development is to understand how close or far is our test formulation from RLD, hence maintaining sink condition is not necessary, as without sink condition our purpose is served. I hope this helps.Sink conditions on the other hand help in maintaining concentration gradient from the core of the dosage form to the bulk of the media, mimicking stomach condition.This is helpful when we want to achieve faster release and complete release .So the type of condition we set-non-sink/sink will depend on the purpose we are looking for