Okay.. now that I'm actually next to a computer I can put this down how I see it.
In RE: to old sites
As has been stated; older sites can have two pressures on them to keep their current version...
1. Their modifications and styles. In many cases it hasn't come easy or cheaply, and at the end of the day, their site has gone x many years without being hacked. As time progresses, systems aren't becoming less secure, they're becoming more secure. So if their websites frontend and backend are both secure, then the only insecurity would be their server. If their server is secure, what's there to worry about? Keep passwords safe and life is sweet.
2. There's a pressure for some of them is not loosing their members to a major upgrade.
Downtime, bugs, unwanted features, new features creating possibly security holes, etc. Upgrading can seem a little daunting and risk.
This doesn't apply to all sites, just some of the ones I've worked with.
That said, I know many admins who will upgrade to the latest version and migrate over all their mods and styles themselves. It may take them months to do, but their site gains functionality in their eyes and that's what matters.
In RE: to the look of vBulletin4.
It boils down to your personal opinion. For me personally, I would prefer to keep vB4 as the default look because it doesn't look too bad.
But from a developers POV, I found that vB3 was much easier to work with. Everything seemed much simplier.
That said, I haven't had a great deal to do with vB4, but at the same time, I'm not complaining.
I would love to see what would happen if they released a vB3 style for vB4 and added it to the standard install... I wonder if we'd see a fair bit of themes cropping up from old times