We chose 'exact' as default because it's the fastest and poses less burdern on DB (which is a concern for many users).
To choose a different default, you can make a very small modification in file :
modules/mod_cpsearch/mod_cpsearch.php at line 442:
From
Code:
$checked = JRequest::getCmd('searchphrase', 'exact');
to
Code:
$checked = JRequest::getCmd('searchphrase', 'all');
or
Code:
$checked = JRequest::getCmd('searchphrase', 'any');
