I am going to partially disagree with Doug on this one. For the following reason.
There seems to be some question as to the effacy of chemical purification with regards to hard shelled parasites. It would seem to me that even MP1 tablets may not be 100% effective under certain conditions. Those conditions arise when the water is too cold or has high levels of turbidity. They can only be removed by physical filtration or destroyed by boiling. Sometimes boiling is impractical, or the contact time for maximum effect becomes excessive. Most effective filters are impractical for small survival kit's by virtue of their size. Under those circumstances there is a place for a filtration straw.
The best way to use is: Fill water bottle, Puri-Tab it. Leave for as long as possible. When you need to drink, insert the straw into the waterbotttle and suck up the water.