Even though this question has been "beat to death" I wanted to share my experience and knowledge about Mackeeper and another click program genieo (genieo is no doubt malware and proof that you need some kind of malware scanner on a Mac nowadays.)
I had Mackeeper installed on both my new Macs. A top spec MacBook Pro 13" (2012) and a Mac Mini custom ordered through Apple with every upgrade paired with a 27" Thunderbolt Display.
I consider my self pretty computer savvy but was a new Mac convert and read all the seemingly awesome reviews on Mackeeper. So I fell for all the ads and most likely paid reviews and bought for both my computers. I did this when I first set them up so I had no benchmark on whether it slowed them down. What a huge huge mistake.
Both my systems were so slow my wife who was still in the PC world said this is why I don't ever buy a Mac, they are so slow all I get is a colored wheel every time I use your computers. She would constantly say I paid thousands of dollars for junk and that a 400 dollar PC would be much faster. I just hung my head low knowing she had a point.
After some quick research I uninstalled Mackkeeper by simply opening up Applications in Finder and dragging to Trash, this iniated Mackeeper's uninstaller which asks why you want to unistall (which you don't have to answer) I simply clicked uninstall. I emptied the trash, restarted and it was like I had just went from 2004 to 2014, my MacMini was incredible, I was elated.
I quickly did the same on my MacBook Pro and the performance increase was even more astounding. No more little Racing helmet icon on top of screen with constantly rotating gear! Alas, I finally had my money's worth in computing power. My available ram went through the roof on both systems showing Mackeeper was hogging and wasting all the memory.
While Mackeeper may not technically be malware it is a horrendous program, a waste of money, and destroys your computer when its installed.
As for the argument you don't need Malware/Antivirus protection I will give you one piece of advice and an example. There's a program called Genieo which is hidden in downloads of other programs as part of a bundle. Geneio pays to have the product put in the bundle of an actual program. This is Adware/Malware and a vicious vile one at that. It will hijack your browsers constantly displaying ads, and eventually take over your whole system.
When you download and run Bitdefender it will identify Geneio and remove it, though you will still have to work to fix your browser settings. My point is now that Macs are becoming more prevalent and have in my opinion a wealthier clientele, they are ripe for the picking since hardcore Mac enthusiasts will swear you don't need any protection. I am not saying you need an antivirus program continually running but it's good to run a scan every so often. Even though Mackeeper will not be identified as a malicious program, virus, or such it is complete garbage.
My wife by the way loves to use my Macs now!!