Or "changed back", since things had been just as simple back in TOS and TAS.
I figured they were developed as a defense more effective against physical projectiles, since they were based on the same technology as navigational deflectors.
It does make intuitive sense that bubblelike shields standing off the ship's hull would be a better cushion against physical impacts... No real sense, perhaps, but intuitive sense.
Yeah, I like your reasoning. Possibly the bubble mode for shields was all-new in TMP, but this was the last time the mode was considered a separate device; thereafter everybody just considered it part of the generic shields.