The technique puts strain on your chords and swells them. When they swell the grit can get so 'far' apart that your range gets really limited.
He also isn't putting the unnecessary power behind his chords, as you may have noticed in some of those 'Welcome to the Jungle' rants or ESPECIALLY the 'PERFECT CRIME' videos; he's much older and the chords just won't take it.
The difference between Axl Rose and say, Stephen Tyler, is that Stephen Tyler isn't pinching or restricting his range...he just lets it flow (it's a blues style of singing). Axl pinches his notes and stays in head voice when he could easily sing in a 'normal' voice; this interruption of the natural air flow also puts a lot of strain on the vocals.
In Axl Rose's mind if he sings higher/cleaner he's putting less strain on his voice because he's already in 'head voice.' Even a song like Jungle is just barely in head voice and could be sung otherwise, it would sound like 'shit,' but, it could be done.
Axl's voice will naturally get more raspy, but it's best for his chords to callous over first; otherwise he'd sound like Brian Johnson after one show or even a couple of songs and struggle his ass off to sing a 'tour.'
I typically sing in a pretty 'clean' voice, but if I know we're going to cover "Jungle"
- then I'll use the technique because I know it's just one 'song.' I'm also much younger and my voice is going to put up with the abuse.
If I planned on singing like that for an entire tour I would be doing SHITLOADS of warm-ups; that is what Axl has done...so, when you hear him sounding all squeeky, it's because he's in REALLY good shape. He probably smokes very few cigarettes, drinks very sparingly, etc.
By Japan he'll be getting close to what you 'want' and his voice will be ready for it.
Axl's the man!! I think the fact that he's squeeky is an AWESOME sign for all of us. 2002 was different because he wasn't just squeeky, his voice was actually weak, he was out of physical shape and he just couldn't sustain it. He's obviously done a FANTASTIC job turning his life around.