i agree. who the hell has the time to fly them all?
For practical purposes I agree. However, who said we have to be practical in this hobby with so many free goodies to try?
I wouldn't mind betting that most people here have at least some aircraft installed to their own sim that they never use & have completely forgotten about. This is no big problem as a few extra aircraft should not affect the sim in any way. A large amount like 497 might be overdoing it slightly.
Using 3rd party aircraft for AI traffic can easily double the amount of aircraft you need. These can easily be tweaked not to display on the menu but the folders still need to be installed or the AI traffic will not work as advertised.
It's often easier to leave them where they are. Deleting them could cause problems with others you do use. Tidying up the AIRCRAFT folder is also the main cause of deleting important default files like default.dp, causing all AI traffic to suddenly & mysteriously disappear. If you're a particularly tidy person, dragging unwanted aircraft into a Hangar subfolder is the ideal solution for organising your 3rd party aircraft. They can then be safely deleted later if no problems raise their head.
PS. In FS2002 the AI aircraft are defined by their title= name in Aircraft.cfg. If this is the case with FS9 I see no reason why the folder names of PAI aircraft should not be changed to whatever you wish.