I run PHP 5.3 on my in house dev box, since it is intended to let lots of versions of vBulletin run (all the way back to vB1). If I was concerned about performance I'd be on 5.4 or 5.5 with some modifications to vBulletin to accommodate that.
You really shouldn't. PHP 5.2 isn't getting updated anymore, neither is 5.3 to be fair, but 5.3 runs better than 5.2 and 5.4 is better than 5.3, you just need to get versions that support it (388 422)
I would upgrade if my other vb lic. would be allowed to upgrade to 3.8.8 without the cost of buying versions I will not use. Or at least a reduced cost to buy up to 3.8.8 with future updates if any.....That would be great.