I think at this point it's mainly just a placebo effect. People get all psyched up about the Quick Health thing, and then are convinced that they can see a night-and-day difference in health update rates, and that this radically improves their healing.
Grid2 is not yet ready for general use by non-testers. It has several outstanding bugs that make it unsuitable for healing (frames bug out and show as black squares), and the configuration UI is very rough and under heavy development.
When it is complete, you will be able to add/remove/customize indicators, so you will not need plugins to have extra icon indicators, a third text indicator, side indicators, or threat.
A raid debuff plugin for Grid2 is already written.
Currently AFK and raid icon statuses are not present in the Grid2 core.
Mana bars are not, and never will be, present in the Grid2 core, and you do not need them for anything. As a resto shaman, you can simply set the "Low Mana Alert" threshold to 50% or so. If you see lots of blue borders (or boxes or whatevers) in your party, Mana Tide will be beneficial. As a raid leader, 50% is also a good threshold for knowing whether you're ready for the next trash pull, and for bosses you should be doing a ready check anyway. The mana bars plugin has always caused significant issues with other plugins and even with Grid itself, due in part to the fact that Grid was never designed to be modified that way.
Finally, you don't need GridStatusThreat even now... this was merged into the Grid core some time ago. Check the "Show threat levels" option in the menu for the Aggro status.