@Mark *********
This.
I am not familar with the city water of Bangkok, but in many a developing-world city the water supply has heavy metals and other toxins leaching into the ground or river source due to improperly disposed industrial water and waste upstream. Boiling does absolutely nothing to remove these toxins. And water softeners add sodium to your intake.
If somebody has recent research about the BKK water supply, would be much obliged. Oh, and the "I'm drinking it and I feel fine" comments are only helpful for short-term visitors. Anyone living here for years can be exposed to harmful buildup of metals in the water supply. It's doesn't slap you in the face right away.