It’s not only your interest, it’s the very reason why fasttracking has been implemented.
Not sure what you are asking. If you need to use mangle rules together with fasttracking, you have to carefully plan them so that the largest part of the total traffic would not need any routing mark and therefore could benefit form fasttracking. If you, theoretically, wanted to distribute traffic evenly between two WANs by means of mangle rules, only half of the traffic volume would benefit from fasttracking. Try reading http://forum.mikrotik.com/t/static-default-route-im-missing-something/119183/9 .