@Reyairia:
Because Arlong was a very serious villain who did major damage to one of the Shs and it would suck to see him downgraded to Buggy's status (Buggy, for earth's sake) for absolutely no reason aside from a lame sense of humor. He's more important than that, he's a more serious villain than that.
and Nami's just went through serious training. just cause nami can defeat him does not make him any less stronger. obviously her history with arlong's crew still is a huge problem for her (such as when she saw hachi again at SA, and just now at the tattoo), this fight would be incredible closure for her.
@Reyairia:
Either villains or villain lackeys, which would include Arlong, Kapoty in Foxy's crew, the Macro pirates (despite the fact that their existence makes no sense), and that random fishman in Strong World.
Let's not forget that Arlong was a pretty big villain since he's the one that caused a SH a lot of damage.
We've only seen three good fishmen and one of them was a former villain so he only kind of counts, another was a flashback character. That just currently leaves Jinbe.
when i mean villain, i meant as a hardcore villain. kapoty was i think in only 1 event. in foxy's crew, only the monkey, the girl and foxy can be considered villains. the macros were also just henchmen of Duval, they played no big part whatsoever. only one merman has gotten several chapters to himself and has done considerable damage to almost the entire crew (at the time) which was arlong. as a race said to be 10x physically stronger than the regular human, i would expect that there would be several more merman villains as a New World merman would technically be stronger than a New World human, making luffy's power increase that much more effectively shown
@Fezzik:
There was nothing serious about Buggy, his fight with Luffy was like a circus act
the PIRATE buggy is a joke. however, buggy as a character is very serious. asmuch as he is comic relief, he is also a part of the story now. for example, assuming he actually became a shichibukai, we know he is useless against luffy. however, the powers and relationships between the two could be determining factors in huge battles/arcs to come (say if he takes out buggy, the marines would send admirals after luffy for taking out the 3rd shichibukai.)