I believe the risk depends on how much you drink. If you drank nothing but stream water for several days, you might get problems you wouldn't get from just a mouthful.

I imagine the stream water is less wholesome if there is a dead sheep lying in it upstream. Or whatever; there is usually an element of risk.

If you have the means to make fire, and you have a mug you can use as a pan to boil water in, purification tablets become optional in my view. I carry some now because I had some available and they don't take up much room.