[TheLastSuperman]

EVVVERRBODDDDIIIIIE GET TO THE CHOPPA! The Don't do it Choppa!

lol joking aside, no need as many have already stated.

Only reasons to upgrade would be:
- Some compelling new feature simply not available in vB3/vB4.
- Due to a security issue i.e. if they simply didn't patch vB4 then you would of course be required then but usually some small form of patch for earlier versions is released.
- Due to PHP incompatibility. For some time PHP5.2 was the golden era for vBulletin 4.x series, as PHP progressed though so did the software and in-turn it's PHP requirements and dependencies (PHP modules required such as MySQLi if you want to use it instead and so on).

With that being said about PHP you need to realize there may come a time when you're either forced to upgrade to a newer version of vB even if it's just a slight number higher such as 3.8.10 from 3.8.7 (as 3.8.10 has the ability to be up-to-date with PayPal's new requirements but not versions before as they were never updated to do so). OR you may need to move hosts if they're running new versions of let's say CentOS and it will only run PHP5.4 with some tweaks if I recall, 5.6 is the default of many installations now if not higher and they're not offering a selection of older php versions (deprecated). So if you're running let's say for example vBulletin 3.7.3, you should be on PHP5.2 or 5.3 at the highest (unconfirmed on 5.3 from me that might be a maybe) because the default 5.6 won't work there will be so many errors the file edits alone wouldn't be enough to retain your sanity.

So it still boils down to the no need to fix what ain't broke because unless you break it by some mandatory PHP upgrade requirement it should run just fine. Sometimes though for example companies, may have a corporate policy to ensure the most recent PHP version is used at all times - that might see a need for an upgrade to a newer version i.e. a jump from vB3 to vB4 / vB4 to vB5 etc etc but it's important to review all the information available and only upgrade if it's required. Also don't forget that every time vBulletin releases a new version they do an announcement on the forum with information relevant to that specific version such as a specific PHP version requirement or similar, always search and reference past announcements for a quick and easy way to find php specifics on different versions.