It has always seemed logical to me to watch every replay in a PBEM game, it provides priceless intel on the movements of your opponents forces and seeing the fire results is a good indication of the strength of units you are facing, it can tell you if a XXX strength unit is a company or battalion for example, also you can sometimes see flanking units moving that are then out of LOS when you opponents turn ends.
However over the years I have played some players who don't watch the replays, one reason I was given was that the time spent watching replays was better invested playing more PBEM turns and they were happy to accept the fact they were playing at a disadvantage by missing out on the intel that replays provide.
So what's the view out there? Watching replays seems a no brainer to me, but playing these games for 20 years has taught me there are lots of differing opinions out there...........