Maybe because he enjoys the new features of FSX that do not exist in the previous versions ?
Some people here are not interested only in graphism quality

Yep, that's it. You're probably right. He's enjoying flying gliders and missions, and looking out through beautiful default virtual cockpit windows over terrain that looks like FS7 and jerks and stutters like hell

I suppose if he flies multiplayer this seems to have been upgraded, so that would be a good reason to move to FSX...
Still, as far as I can see, both FS9 and FSX as improvements on FS8 ARE mainly about the graphics, though the missions are something cool, and I'd love to try out the gliding. There's not much new in there (as far as I've heard) for big-iron drivers, or serious pilots who want realistic atc, flight models etc etc. These have remained unchanged. It's all about low, slow and looking at the view, rescuing people from oil rigs and so on, it seems. And on a great rig, it looks great, so why not upgrade. But on the-day-before-yesterday's modest rig (and not everybody can afford the best, nothing wrong with you if you have a modest comp) it looks and flies terribly. Or at least, I imagine 8 - 15 FPS flies terribly. And if you've only just got FS9 running okay, or even that stutters and lurches, then why dive into FSX. You're just shooting yourself in the foot with a panzerfaust.
No disrespect intended to Alrot with this post!!!! Just an opinion and some advice... I'll stop now, though. Not everyone shares my views on FSX I know, or has such base motivations for using the sim. The main problem I am having with not upgrading to FSX is curiosity and envy, but so far I'm keeping them under control
