there really is no diminishing returns on any of the 3 expertise bonuses now.
however, once you are within 2% of the maximum expertise, it becomes somewhat beneficial to swap out a few mods for higher main stat.
And thats what my concerns is about, cause where can you get better mods than the ones in pvp gear? The only mods i know about are the ones dropping in operations. Thats why im wondering if doing operations is more beneftial than it should be. i guess it comes down to how much better the replacemnets mods are. If you get 200 more endurance it will give you an aditionally 2k health! Sounds beneftial enough to me, and when those mods also have strength (if ur a sentinel) and other bonuses it sounds like its worth it.