My understanding is that it will take a while for symptoms of a giardiasis infection to develop while you can die of dehydration quite rapidly. If you need to spend a lot effort to collect fire wood to boil water you may be loosing more water than you are gaining. Also as you boil water some of that water is being lost in the act of boiling. Not a problem at home on the stove where you can turn on the tap and get more. However, if you spend hours collecting the dew from leaves in the morning you will want to keep every drop. Of course, if combustible materials are plentiful and easy to collect and water is plentiful then by all means boil away.