here's the way to think of it - your rate of increase functions on a curve. the closer you get to the cap, the slower the rate of increase. WE and playing time factor in - we all know this, this is well understood. well, the rate of decrease functions on the *exact same curve*. so, when you are looking at a situation where 20m would net you 2 points - say, when you are 5-6 points out - then 0m is going to net you maybe -0.5 points or something. if you are 50 points out, and 20 points might net you +22, then 0 might cost you 5 or whatever. its the same curve.
its technically true, when admin says, the figure guaranteed not to drop you is 7m or 3m at lp/per. but, that is GUARANTEED not to drop you - meaning, at 1 work ethic, with 0mpg, that is what will not drop you. but, for most players, that number is lower. and when you are near maxed, even if you are dropping, if its so slow you'll never notice - then who cares? i will often 0 out low areas, even with like 5 points of growth, when they are off-cores, to get more time for cores for youngsters. its all about risk/reward. you risk little, and the extra 7 minutes in some high potential cores, is worth a good bit. so, i don't think this conversation is just about maxed categories - where 0ing is a no-brainer unless you have nothing worth practicing - but its also about low categories in general, where its often worth 0ing out to spend minutes in some category where you are still in the rapid growth part of the curve.