One thing that I really wanted to see was the bounty hunters. Zoro was supposed to be one, and from that I always though we would see more of them, but maybe they are only outside of the GL? And if there are almost no bounty hunters, why have a bounty at all? Do the marines get it as a bonus if they catch the guy?
Oda has never paid that much attention to certain things in the story. Take sea warfare, like meeting other pirates and fighting while both ships are on water. The only time we've ever seen that was Wapol as I recall, lol. Bounty hunters like persay one person romaing around trying to catch pirates is also part of what the story does not really include, as those thugs shouldn't form a threat to Luffy and the gang in the first place.
When I was thinking of Bounty Hunters however, it occurred to me that they have been playing quite a part in the story, even if it's not in the definition I gave earlier.
Remember Baroque Works back in Whiskey Peak? They served a much bigger purpose, as we saw it become evident later on, but the first time we saw them they were shown to deceive and catch pirates.
Then, we got to Water 7 and the Franky Family was going about there. They were even less of bounty hunters than the BW guys were, but think back on it. They harrassed the SH's completely because the law wouldn't help them anyways. Not sure if it was stated that they did that more often with other pirates though, and I'm not sure if they actually ever caught pirates for bounties, so I won't call them bounty hunters. Still it's something a bit similar.
Lastly there were all the guys on Sabaody Archipelago. Lots of people were caught to sell in the auctions, and even though mermaids and all played a bigger part and the SHs were too busy with bigger stuff (giant robots 'n admirals and all) because that's what we want to see, we did see stuff like pirates being sold at the auction, if they happened to have been caught.
So yea, over time we have seen a few examples, not even including Jean Ango. But as I stated before bounty hunters and a lot of other things just don't play a big, explicit part in the story aside from these examples.
EDIT: Oh also to get into your questions, bounties also indicate the threat levels of the people on the posters, and notify the world of them. Higher bounties also get higher level marines to hunt down the stronger pirates, but I'm not sure if they get the bounties of pirates they catch. If I had to take a wild guess I'd say they don't, as it is their duty.