I don't think 3.9 should be consider a major version step toward 3.1.
It is a small upgrade because of the libs, i think, no more.
2.1 to 3.0 was a major system upgrade, introducing new concepts in the OS, but 3.1 to 3.9 don't change anything important, but eventually add more bugs :p
I usually call my 3.1 system on my main amiga a 3.7 version, because it is so heavealy patched that it is not a mere 3.1 but nothing like 3.5 or 3.9
