I'm convinced it hinders. But I stress I'm by no means I'm an insider, just trying to make educated guesses like everybody else...
According to a BT patent application for an ADSL DLM (which may or may not be the current DLM), each line is categorized, based on errors and frequency of resynchs, as being 'very poor, poor, acceptable and very stable'. That patent goes on to describe how, in order to stay level in terms of target marging, the line needs to be 'acceptable'. But in order to win a reduction, the line has to be 'very stable' - i.e. better than it was to begin with.
Now, if you sit back and suffer the consequences of a high target margin, your line probably connect at a slower speed so you ought to get fewer errors, and may well be judged to be 'very stable' in which case you'd get a reduction. But if you tweak it back again to the old connectioon speed, it will have the same error rates, which have already been decreed to be merely 'acceptable' and so the margin won't move.
I stress that Im guessing. I've no way of knowing for sure whether that BT Patent describes the current DLM, or some other DLM. But it makes sense to me.