Best reason for par and for not changing it from 5 to 4 arbitrarily:
Last year at the USGA Senior Amateur Qualifier on the Witch Hollow course at Pumpkin Ridge, the Oregon Golf Association saw fit to change the par on the 470 yard 14th hole from 5 to 4.
As we walked off the tee after our drives, one of players I was playing with complained about them changing the par. I gave my usual response that par doesn't matter, only the score after 18 holes. I got to my drive which was probably the longest one I'd hit in quite a few years on that hole. From the rough, down-hill, with a good tail wind I proceeded to hit a 180-yard 8-iron which bounced a few times and rolled into the hole for a 2. I promise that par mattered very greatly to me at that moment.
Best reason for changing par from 5 to 4:
It gets into the heads of the players. Even many of the best ones in the world who whine that they won't make birdie on a hole when the USGA makes it a 4. They change their entire playing strategy based on it. They feel they have to go for the green in two, it is no longer an option since it is a par 4. While the USGA and others will continue to do it, I think that there is some realization that you have to look at how the hole sets up before changing the par.
Favorite story about changing par:
John Huston set the tour record for most strokes under par at the Hawaiian Open a few years ago. A year or two later, par was changed from 72 to 70 on the course. This was done by simply renumbering two holes from 5 to 4. The holes played the same length. That year, one of the TV announcers went on for a while about how the 13th hole had gone from the easiest hole on the course to the hardest.
The average score was probably still about the same, but if you measured it against par, it had done so.