Just by coincidence I'm experienceing the same isue with the door on my 88 16' Rebuild. The problem is not caused by the saw kerfs at the bottom of the door. Someone somewhere along the line tried to fix the problem with these cuts in order to allow all of the moisture out of the door that was getting in through a leaking window or door handle. This is just my opinnion anyways. This was certainly the case for my door because there was no saw kerfs or drain holes.
The interior components of the door are 2 -3/4" thick strips of osb at the door handle attachment point, and then what I believe is MDF (medium density fiberboard) aka "Black-Board". Once wet the MDF will
NEVER dry out, saw kerfs, drain holes or not.
A very interesting door rebuild method was explored on this board by board member
Con.
See the thread that contains some discussion about the door repair and some modifications
Here. Also follow his link to his photos that show the repair in quite good detail.
My plans are to follow Con's steps but to use 3/4" mylar backed foam insulation in the voids, and then glass the 2 door halves back together. I will be drilling drain holes in the bottom to let any water that may end up getting past my re-sealing of the
windows and handles.
Here is the view of the "guts" of my door after splitting it open.