I was actually thinking more in terms of security and support.
PPC architectures are not very well supported nowadays, and haven't been for over 5 years. Perhaps longer. Sure, you can still run your machine on 10.5 and it might run really well still. That's no reason to trust it for mission critical tasks. God forbid if someone was using a OS X Server 10.5 and users trusted it with their credentials.
Vanity was pretty low on my list, but I understand everyone's priorities are different.
Not sure I trust Apple with longevity. Windows XP from 2002 just hit the dust and I've got a decade out of RHEL. For something from 2007 to be a write off is not good IMHO. I thoroughly regret my MBP purchase for theae reasons.