It is a fair point that I should have used the uninstall option in the app itself. We all know that now. And in a theoretically perfect world no cleaning or deleting would be necessary. - But.
I have been working with PC machines since they were invented. I can still remember PC Dos upgrading to version 2.11 which made it possible to create folders on the floppy disk! Now we suddenly could store more than 99 files on a floppy!!
So I do not scare easily when I encounter imperfect software. Also, over the cause of time I have learned that the uninstall option in a particular app (called program in the old days) cannot be expected to be reliable either. The uninstall-program code may also contain errors and omissions. The same goes for the operation systems programs, and of course for apps like CleanMyMac. So nothing is perfect (until after the next upgrade…) - My 30 years with PC computers have mostly been with Microsoft operating systems. But I also worked with the very first Apple computers and later some time with IBM OS/2. For the last five years I have worked with Apple only. So I have seen debates like this one many times before. (The stupid Apple IOS/Android war is a re-run of the debates over PC DOS and MS DOS back in the old days. We also had the same debate about IBM OS/2 and Windows.)
My first iMac is a late 2009 model which I bought in 2009 when we used Snow Leopard. Now it is running the latest/final version of Mavericks. For the most of this period I have used CleanMyMac and CleanMyMac 2 for regular maintenance. On total the CleanMyMac(2) has deleted about 1,5 TB on my 1 TB harddrive. And the iMac is running just as quickly now as it did on day one. The deleted files are not just those in the trash can. It is also a vast amount of language files, cashes, broken references and large/old files (if you ask it to do so). - So the statement that CleanMyMac is useless is utter nonsense.
As it was suggested I had a look on some of the posts in the discussion forum about CleanMyMac. - Yes, some users claim that CleanMyMac has caused them some problems. And many posts just want to know if it is “the best” and why it costs money. - Some posts are un-reproducable cries of panic. And probably some are quite true, although to poorly described to provide any kind of certainty.
Obviously gurus in this forum can (sometimes) do better themselves than a CleanMyMac app can. But my wife cannot! And I cannot or won’t spend my time as an operating systems guru any more. I am 63 years old and have retired. So for ordinary users I still find CleanMyMac to be an easy, reliable solution to the everyday problem of deleting junk and other stuff from our iMacs. - I have deleted apps with CleanMyMac quite a few times. The cleaning is not just about deleting the app file. CleanMyMac finds all the related stuff as well and show/ask permission to delete it. The problem with DuetDisplay is my first problem. And I don’t really think that CleanMyMac is to blame here. The problem is that one app is allowed to change the functionality of another app and its data. DuetDisplay has been allowed to modify data belonging to Yosemite.
I thank you for your comments and your time. Although I have do disagree with your warnings about CleanMyMac from a practical standpoint. It works better than I could have done myself. Automatically every 2 weeks and on request. And my wife is happy!