Others might have technical reasons to cite. Some people classify it as a "PUP" (potentially useless program). Personally, it's about their marketing and distribution methods.
Apparently, they're willing to pay people for distributing their product as part of fraudulent downloads. By fraudulent, I mean either the "You need xxxxx to view videos on this website. Download now" pop-ups that don't even deliver xxxxx, or in bundles from download sites that include unadvertised "bonuses." It's common to get both adware and MacKeeper in those downloads - perhaps that's just guilt by association, but since MacKeeper may also be advertised by that adware...
Then, there are the persistent pop-up notifications to pay for the software. If you knowingly download an app on a free trial, it's fair for that developer to periodically ask you to pay - if/when you open and use the app (notifications are part of the app itself). However, if you never asked for the program in the first place, haven't opened or knowingly installed it, you still get reminder pop-ups, and you have a hard time making those popups go away...? No, not ligit. Further, those popups come from adding an item to System Preferences > Users & Groups > Login Items, or System startup items... you're running apps/processes you did not choose to run. That unwanted "reminder" app can indeed hurt overall system performance and degrade the user experience - maybe an imperceptible amount (on systems with plenty of resources), but on systems that are light on resources and/or already have other issues, it might be the straw that broke the camel's back, leading you to pay MacKeeper!
Then the user has a choice - pay up, or uninstall. Though it's easier to uninstall MacKeeper now than in the past, it's still tricky, as this article at MacWorld describes:
http://www.macworld.com/article/286...how-to-uninstall-mackeeper-from-your-mac.html.
So even if MacKeeper was the best thing since sliced bread (and there's hardly agreement on that), I'd be looking for a competing product that used different sales and marketing methods.
https://en.wikipedia.org/wiki/MacKeeper will have further info for you - not that it takes a position one way or the other, but there are plenty of links for you to click.