The idea is that the male is a normal person without anything memorable. Anyone could be the guy.
Some Anime and manga do it much better than Kannagi though, like Amagami. I remember alot of details of Junichi Tachibana because he was such a well done "everyman" protagonist.
Kannagi was pretty different, a lot less fanservice, and romance and a lot more laxed, off-beat, day to day stuff than your regular harem. That's why it's kind of weird that the main guy is so forgettable.
The idea is that the male is a normal person without anything memorable. Anyone could be the guy.
Which is something I always hated about a lot of animes. In fact, the "main girl" of any harem anime is also generally a generic, inoffensive, totally forgettable lowest-common-denominator (since, if we're hooking the This-Guy-Is-You character up with a girl, then if she has any personality traits at all, they might be personality traits random audience members might not like!) that I pretty much dismissed the main couple by default for years. It was always the side characters that had the actual personality that carried the show.
It's part of why I liked Haruhi so much: Kyon was a jackass taking the piss on everything as it came. Even if I didn't quite like how much he focused on ogling Mikuru when there were two other girls I liked much more, personally, at least he was a proper character in his own right, rather than just an audience placeholder.
Major reason why I loved Gate so much. Itami is a lazy otaku who works in the military and was pretty much despised by all of his co-workers, but when push comes to shove, he has the biggest balls around. Its a kind of crouching moron, hidden badass kind of dynamic, but instead of him having two different personalities, its just him doing what he always has done but in a different situation. Funnily enough, I consider Itami to be more relatable than a vast majority of these so called everymans.
