Man has always messed with mother nature and not always or even very often had bad results come of it. We've chosen not necessarily the healthiest animals to breed thus bypassing natural selection so that we could have cows that gave more milk, sheep that gave softer and more wool, chickens that laid more or grew so fast that their legs broke. Look at the miniature horse. I can't see any purpose in making horses smaller or bigger but there's a demand and we've been able to fulfill people's needs through selective breeding. Now we have cloning. Not a problem at all. If you had the cow that gives twice as much milk but cannot seem to pass on that trait and in her lifetime will only give birth 6 or 7 times don't you think you'd jump at the chance to clone about 1500 head of that girl and keep trying to find that dominant gene to make ALL cows give that much milk? Now certainly cloning a mammoth doesn't have any altruistic motive other than to see something that hasn't walked the earth in a few thousand years but it's really no different than intentionally breeding cats with short legs or miniature horses is it?
