I see no reason why you couldn't use it this way. Just make sure to remember that this container is contaminated until the next time you wash/disinfect it.

Or with a container like a stainless bottle, if you ever stop and boil water in it, say for a spot of tea or for a freeze dried meal, the bottle should be sterilized if you make sure to let it heat all the way to the mouth of the container.

I know, I know--if you can boil water why use something like a Lifestraw? Well, if fuel or time are concerns, something like a Lifestraw has its advantages. Every product has strengths and weaknesses. The trick is to pick the one (or combination of products) that best meet your requirements.

Theoretically, the bacteria count could be increasing in your container if there is organic matter in the water to feed them, but any good water purifying product should still be able to handle it IMHO.