Then there is also the question of bringing back recently extinct species or prehistoric ones. Recent ones could be interesting : as they are extinct, we can assume they will not develop to pests and thus we can do something about the species diversity problem.
Prehistoric ones : It is all about bringing in a species somewhere it was not before.
But there is one advantage with those huge prehistoric animals like mammoths : they are huge. This means we can easily spot them and kill them if they become pests. Did a huge animal ever became a pest ?