I am way late to this party but since I went thru the pain of understanding it, I thought I would share what we did.
It is indeed a watered down limited solution at this point with no integration to newer versions of the powerpoint desktop app (Any MS Office Powerpoint Desktop App newer than 2013 doesn't work), as all those features were removed after 2013.
You can't add slides from the newer MS Powerpoint desktop App, have to do it through IE only browser (Other browsers aren't supported) publishing is not available in newer Powerpoint Desktop App. You can however trim down the uploaded PowerPoint Presentation to fewer slides by selecting only certain slides during that step (Which really is the only feature that I can say still exists in the whole process).
On the other side of things getting slides out of the Slide Library is virtually impossible using a newer version of the PowerPoint Desktop App...
So what you have to do is use the 'download a copy' button for each presentation that you want a slide from and download each presentation locally... (Yeah great way of doing things right Microsoft... << Insert Sarcasm HERE >>)
After you have all the old presentations locally and the new 'in-progress' presentation open in a newer version of the PowerPoint Desktop App all you have to do is use the 'Reuse' (button) feature to browse to each local copy of the old downloaded presentations and select the slides you want, then move on to the next old presentation and repeat until you 'inserted' all the slides from the old presentations you want.
Afterwards you can use PowerPoint as normal to modify the slides and such that were 'inserted' into the newer 'in-progress' presentation.
Not nearly as elegant as it once was and really the only thing it gets you is the feature of trimming down your presentations when being uploaded, but it is what it is.
I tried mapping a folder location to the Office 365 Slide Library location to see if I could trick PowerPoint into letting me do it that way, but got an error saying 'You cannot use an Internet address here...'
Such a bummer, because it was a good centralized integrated way of managing slides and presentations, but that is the only option I see, short of staying in an all 2013 environment of course...
I should mention that after we put the process in place we created some basic self paced training videos and uploaded in 0365 Stream for users to walk-thru the process b/c is not integrated. We thought that might help eliminate some of the end-user frustration with this issue.