I've used some pretty intense goop for races, rinsed the bottle off and then spent the next day driving home.

Those bottles can get a bit nasty over time. If you don't have a completely clear bottle chances are you're missing small amounts of stuff living in the threads for the cap and around the valve. Even washing with a brush you can miss those.

When I was running a team this meant washing 20~30 bottles a day. This can get quite time consuming.

I would bleach bottles once or twice a season and then wash them to make sure no-one was going to suffer upper GI distress because of bacteria lurking in their bottles.

I still do this with my own bottles and things are usually fine.