Personally I have mmm trouble/style with a lot of ammo-preservation, and at most of games I prefer common and low level stuff or better infinity with more slow and accurate usage to progression.
This way, free shielding give me a big fun as free stuff itself with limited effective usage.
But agree with inactive disappearing, because it's funny when you reviewing level for secrets and found old shield used at level start. (But it's also harmless thing, and may be useful and funny enough at his current state. Simple timer can be not so good, but timer based on last damage time may be more difficult to implement and just over-featured)
My first reaction almost same as your, but over walkthrough i feel it's just already good enough.