not every companion needs to have a tadpole inside there heads why limit yourself
If tadpoles are used as an explanation to every anomaly (super jump, super strength, super push, mindmeld with companions) and central theme of the game revolves around accepting or rejecting it's power, then yes, every "Companons" needs to have one. Just as if BG1&2 were coop focused game then everyone would need to be a child of Bhaal, to be central to the story and conflict, and face the same dilemma. Tadpoles seem to fill the role of Source from D:OS2 so well, it is difficult as of now to expect anything different.
As far as narrative devices go, it is not a bad - gives every player a unified objective, while allowing them to be their own character. As mindflayers abduct ton of people, it is not difficult to imagine that all of the survivors would be infected.
Traditionally, this pressure applies to PC only to give them overriding objective, while allowing for a range of characters to be roleplayed. Meantime companions act as sound boards for game's themes, world and players choices... but it ain't your classic RPG, it's Larian RPG.
