'll admit bias because I suspect that he'll be one of those who are 'not what they seem' in book 7. I'm not entirely sure that his 'I taught him' in HBP with regard to Snape wasn't a boast. We'll find out next year. ;)
That aside, maybe Slughorn doesn't think he's doing harm. He could believe he's a force for good, identifying the 'stars' and 'helping friends' but I suppose it depends on how you'd feel about someone with inferior qualifications getting a job you wanted because of who they know. (As you might have guessed, I've a particularly nasty bunny biting my leg). Perhaps we should agree to disagree?
Good question though.