This is something that seems to have changed in WoW 3.3, that was not evident to me on the PTR during Aloft testing.

The nameplate for the player's current target is not sensitive to the mouse. This is true of Aloft's nameplates, as well as Blizzard's default nameplates (therefore it is not an Aloft defect). The 3D model for the player's current target remains sensitive to the mouse (including when clicking through the nameplate, which can make it seem as if the nameplate is responsive "sometimes"). This seems to be something Blizzard has done.

Since this was an undocumented change in prior behavior, I reported this to Blizzard as a bug some time ago (the day after the 3.3 client was released in North America; the thread is here, though if it is not tagged by Blizzard it may poof at some point). I have not seen any "official" sort of response from Blizzard. My personal suspicion is that they do not consider it a bug (or it is of such low priority that it hasn't been addressed yet).

As a click-targeter, I now find myself habitually clicking away from my current target, "selecting" it with a mouseover, then interacting with it (without targeting it first) via the nameplate using a right-click. As long as the unit is not targeted, mousing over it and right-clicking works just fine. This is not optimal for initiating combat, but seems reasonably convenient for vendors/questgivers/etc.