Would depend on the pathogen in question, how its on the lettuce (and indeed what kind of lettuce), etc. In this particular outbreak, it looks like the CDC's stance is "not really".

But there's a lot of ambient food poisoning that can be prevented by washing lettuce, especially in reducing cross-contamination (the lettuce wasn't contaminated, then got exposed during the cooking prep process, etc.)

I'd be surprised that washing lettuce prevents the second, but I'm open to evidence that it does.