I'm not sure I completely follow what you're saying.
It seems your grievance is that your section does not count Single's league matches towards NTRP, and you want a national rule to force all sections to count all leagues towards NTRP.
How does that affect me in Norcal? Well, mabe in Norcal we don't want Mixed results to count for NTRP, because over here we don't believe Mixed is real tennis? Just kidding... kinda... point is, if there was a national rule such as you suggest, it has implications on other sections also.
Ok, so you might say, allow Norcal 'override' that rule, if we here feel a particular league shouldn't count. To which I say, well, if you in (whereever you are) feel Singles league should count, then why don't you and all the others who feel the same way just pressure your specific section to make it count?
On a different note, and about your concern that a self-rated 3.5 could just play 4.0 singles forever without ever getting bumped up... what do you care? There is always going to be a small percentage of sandbagger players who get their kicks out of beating weaker players, and they are always going to find a way to bend the rules to achieve their sandbagging dreams. Just accept it and move on. Most of your matches are going to be competitive and fun, and that's really the only point of NTRP.