But would the guy on the motorcycle have any kind of case? And if so, on what grounds?
That's up to the judge to decide. There is legal precedent in both directions. Either way, if you are sued, you are going to court. Even if you win, you're going to have to have to countersue to get him to pay your attorney fees. And then even if you win that, you're going to have to hope he actually has the money AND pays it. As anyone will tell you, winning a lawsuit is only half the battle, the other half is actually collecting the money. In some cases, the agrieved party can ask the judge for an injunction preventing distribution of your movie. In a best case scenario you will lose a lot of time. Even if you think you have an iron clad case, EVERY attorney will tell you that there is a chance that you may lose. Where the trial is for instance can make a huge difference. In los angeles or new york where the judges are familiar with entertainment law is a very different case than in paducah kentucky. (nothing against paducah)
There are shows like jackass and borat that take the chance, but they do pay for it in lawsuits and paying off people. If you're going to skip releases you have to set aside money for that, or get specific insurance for it.
Getting a release isn't typically hard and if the person is very resistant to signing one, then that's the kind of person who just might sue you when they see their picture 40 feet high on a billboard. Asking someone to sign a release is also respectful and a basic courtesy. Most stock guys I know carry a release and a pen in their pockets.
In the end, not getting releases is a matter of calculated risk. If it's a crowd at a football stadium from across the field, you probably don't have to get them to sign off. If it's a guy on a motorcycle in a third world country who most likely will never set foot in a movie theater you might be able to swing that too, but just understand, no matter how small, it is still a risk.
You should consult an entertainment attorney about specifically what you want to do, perhaps there's a get out jail free card for documentaries