This conversation has been had many many many times here, just a search will bring them up. The beauty of CI is that it makes NO difference what version of PHP you use. If you want destructors then chuck them in. Remember that models and libraries are only classes, so anything that PHP 5 nativley does, you can still do.
As Derek Allard says in his blog post, there really is no benefit other than a tiny speed improvement by reqriting the entire core. A fair chunk of that core already has PHP 5 capabilities via Base5.php so I really cant see why it would matter at all.
A framework is a base for you to work with, what you put in that framework is entirely up to you. If you want PHP 5 in CI, use PHP 5 in CI! I work in both all the time, and have not had a problem wither either.