3.0L PCV

Just a question out of curiosity.

I have a 3.0L Mitsu engine in my 90 Caravan C/V. The engine itself has
192K miles on it.

I do not believe the PCV valve has been changed since the dawn of
recorded history. It was still pulling a vaccuum but not nearly as much
as after I replaced it.

The curiosity part, what exactly can having a plugged PCV do to the
engine? I know what it is supposed to do but I have never really heard
what happens if you leave it plugged up.

After that, I changed them in all my cars... (and who's idea was it to
hide it on the 98 3.3 like that???)

Chas