MySQL 5 is proven to be slower than MySQL 4, and WordPress doesn’t require any MySQL 5 specific features to operate.
MySQL 5 has performance improvments mostly for InnoDB which WordPress does not use. MySQL 4 is faster at MyISAM which the is the more common db format (and used by WP).
Also, they are insisting that servers run PHP 5.2 minimum for WP 3.2
While PHP 5.2 is faster than PHP 4.4 (and PHP 5.3 is measurably faster than 5.2) it’s not very hard at all to support PHP 4.4 In fact I’m sure they are going to have to go out of their way to force PHP 5 to be required by actually REMOVING simple code that helps PHP 4.4 already in WP.
So why not keep software flexible when it’s easy? The reality is this is probably to help keep reports of security problems with WordPress down by forcing people to keep their servers up to date.
Personally I don’t think that is WordPress’s business or responsibility but I guess people have a choice which software they want to use, especially when it’s free.
ps. You know what else just dawned on me – Matt has data on a million servers that run WordPress when they “phone home”. This doesn’t bother anyone?