Having water is more important than having food. You can go for weeks without food. Without water you are gone in a couple of days at best. I agree with the other posters that you could pick up something that can´t be cured or make you sick quickly when the water is contaminated. If you´re really sure that you´ll have access to safe water the purification can be omitted. When there is a doubt, playing it safe would be preferable. For me the benefit of the tablets outweigh the disatvantage of bulk and weight (which can be negleted anyway).
IMO a water container can´t substitute the tablets as long as it´s empty. It may help you to collect more water but that won´t make it safer.