Also, the arguments for support of old platforms miss an important economic point. The different in cost in moving from 4.5 to 4.6 OR from 4.5 to 4.7 are an order of magnitude larger. It's a MASSIVE effort, as you have to upgrade twice, and one of those times stabilize a system (and associated modules) without any material benefit to the site. That's a terrible waste of time. It's much cheaper just to stay with current releases, absorb the annoyance that comes with frequent upgrades, and avoid all the pitfalls I have laid out already.
Upgrading from Drupal 4.5.x to 4.7.x is not much of a pain really, in case you use core modules only. In fact I devoted a big chunk of my time around christmas to fix bugs in the 4.5 -> 4.7 upgrade path. The core is ready for you to upgrade from 4.5 to 4.7 directly, there are specific functions to help this migration. Drupal.hu still runs on 4.5 and is going to get an upgrade to 4.7 soon. Thanks to the work put into the upgrade system, the data migration is just a click of a button. Of couse we add new stuff to the site and reorganize existing functionality, and that takes time. Also if you use custom modules, you need to invest your time into it, but core supports the direct upgrade nicely. Gabor