AFAIK, the Data Manager Auto-Generates a Data Load Script for the user but if you want to edit it, the change is permanent and you cannot go back to using the Data Manager with that app. However, I use a work around for this by creating a new app, then letting the data manager auto-generate the section I want to add. I then unlock that section and copy/paste it into the original app. It works but it is a total pain to do and not recommended because if you have duplicate field names you have to fix them all beforehand.
I use load editor mostly because I feel I have better control on things. But those who don't know scripting may be using data manager. I also had similar experience when I tried to use both and found some syncing issue between both, I came back to using load editor only. I think its been long time I haven't used data manager. But one of my colleague uses data manager and he is quite happy with it, he doesn't need any significant transformation while load and spends more time in slicing and dicing data in visualization.
Thanks for your feedback. And yes, data editor is quite often my first choice.
Although having a DBA background in my personal history, sometimes the features of data manager are sometimes quite useful and very reasonable. And one really good reason for using the data manager is the control support for joining data objects.
Working with load editor having several data objects selected then they will be automatically joined.
However I believe it would improve a lot if both worlds could be mashed up, because looking on the growing capabilities of data preparation and having a preview of data value distributions prior editing any report, this is a great support and help even for experts.