Zoro was a bounty hunter. Even though that was never elaborated on much in the manga, as Zoro joined soon after he got introduced, but in the anime there are a few flashbacks that bring this up. Then you also have his 'kouhai' Johnny and Yosaku, who were likewise bounty hunters and were quit proud of it too. This was obvious in the manga, but again this was elaborated more on in the anime.
One of the main occupations of Crocodile's former organization, Baroque Works, was bounty hunting. The entire population of Whiskey Peak consisted out of bounty hunters as it was their primary source of income.
Then during the Corrida Colosseum tournament there was a cactus-like character, I forgot his name but he was nicknamed the weapon thief or something, who even 'thanked' Luffy for raiding Impel Down and setting loose all those convicts as he made a killing hunting them down. Two of the other participants, Jet and Abdullah, I think were originally bounty hunters too.
Just before the time skip we saw several people trying to capture the Thousand Sunny to cash it in.
The truth is bounty hunting is a highly lucrative business as long as you have a sense for adventure, are reasonably strong and don't mind the danger. Oda might not have given it that much attention, however there is little doubt that bounty hunting is a rather popular activity in the OP verse. Of course the higher a bounty is, the lower the amount of people will be that could possibly cash it in. On top of that most of the ones that could do it, would most likely already be allied with someone (pirate, marine, RA) which would make it impossible for them to receive their reward. But what should the WG do then? Stop raising bounties after a certain point? That doesn't really make much sense as the most dangerous individuals are worth nothing?
Bounties also have other uses. They give an indication to people how dangerous a person actually is and it might make the one who has a bounty placed on them more nervous. There have been several instances where people, who had a bounty, started to become agitated because of it as even if you can beat all of the hunters, it becomes nerve-racking if you are harassed endlessly by them. Good examples of that are Robin and Duval. When Kizaru arrived on Shabondy there was a regular guy who tried to shoot him as he hoped it would make him famous. There was of course no bounty involved, but there are many people who reason the same. When the Strawhats where strolling around on Shabondy pre-time skip, they were endlessly pursued by hordes of bounty hunters and one of them even explicitly said that if they could catch just one of the Supernova's, they would be set for life as they were worth over a 100m.
Even if it is likely that some bounties might never be cashed in, they are certainly not without merit.