Dave Crow, from Triangle Universities Computation Center (and from whom we got the Heath Clock materials) presented his White Paper on PDS liabilities and limitations.
The PDS was invented way back in the beginning of the OS days. When OS/360 was being architected main memory was small and expensive, so the designers of OS/360 made the basic assumption that I/O was free. They made a large part of the operating system transient, which is to say operating system code was loaded as it was needed. They originally considered having each operating system module live in a separate sequential dataset, but there were two big problems; it was slow, and it was wasteful of disk space. They designed the PDS as a write once, read-many-times load library. It was not built for updates.
As you know, the PDS is used for many purposes beyond its original intent. Dave's White Paper discusses in detail many of its current liabilities: