For me, I open up a target master presentation, so to speak, which I want to
add content slides to.  Then I open up the smaller presentations, one at a
time, which I am assembling content from.  I then use the copy and paste
technique.  Even the rehearsed timings, if you have any,  seem to transfer in
tact.

You could of course open and delete all but one slide save as etc OR use
insert slides from files to make one slide presentations - both would be
pretty laborious.

OR you could use vba to do it. Try this (ON ACOPY) see if it does it for you
If you have 2007 change all the .ppt to .pptx:

Sub singles()
Dim i As Integer
Dim osource As Presentation
Dim otarget As Presentation
'make a temp copy
ActivePresentation.SaveCopyAs (Environ("TEMP") _
& "\tempfile.ppt")
Set osource = Presentations.Open(Environ("TEMP") _
& "\tempfile.ppt")
For i = osource.Slides.Count To 1 Step -1
osource.Slides(i).Cut
Set otarget = Presentations.Add(msoTrue)
otarget.Slides.Paste
otarget.SaveAs (Environ("USERPROFILE") & _
"\Desktop\Slide " & CStr(i)) & ".ppt"
otarget.Close
Set otarget = Nothing
Next
osource.Close
'remove temp copy
Kill (Environ("TEMP") & "\tempfile.ppt")
Set osource = Nothing
End Sub