I've read that ingestion of fuels is a big problem in places like South Africa where fuel is often placed in drink containers. Apparently one has to bring one's own container to purchase fuel. SA has a lot of poverty, and drink containers are often plentiful and already on hand.
I wouldn't be surprised if systematically alternating between using a bottle for drinking water and the same bottle for fuel would cause some health issues.
Accidental sipping from the wrong bottle ONCE isn't quite the same thing.
Yes, I've also used PET soda bottles for fuel (clearly marked, mind you). Do I recommend it? No. Do I want to make it a regular routine? No.