The function is nice and allows easier MOD injections, but currently the array is mostly added directly inside the function call, so it can not be easily modified.
Example from viewtopic.php
a MOD that wants to add its tables to the query needs to be inserted into the array by inline editing.
When the code would be:
The MOD can simply add a $sql_ary['LEFT_JOIN'] = mod_extend_array(); before the $sql = $db->sql_build_query('SELECT', $sql_ary);