First reason I would do it because with some of the addons you'll have some "airports" as part of the package, what you'll never use.
Example : Cook Straight AI ships. You got like 20 habours around Marlborough which are recognized by FS2004 as airports. So annoying to have Kenepuru Head and Portage in my airport list.
The other reason: I have my favourite addon airports. I'm absolutely sure that I will start every flight on one of them, so I want to remove the factory default airports from the list, this way I don't have to scroll 2 pages of useless (from my point of view) airports before I find my loved ones.
I've done the same "cleaning" with my aircrafts. I have only 3 aircrafts. I think there's no reason to have more than that. I haven't heard of pilots flying a Cessna, Boeing747, Bell, F16 on the same day. You learn to handle a few aircrafts, you can fly them well and realisticly. End of story. I also removed all AI planes from the aircraft list with BBQHIDEAI tool so its clear, only contains three aircrafts with KZ*** tail numbers

. I'd like to reach the same cleanness with my airports. At least with the New Zealand ones.
Some people think that FS9 and FSX just games. I think these softwares are very good opportinities for the people who don't have the chance, money or time in the real world to fly so I try to make it as realistic as I can.
So...that's why...
