I find it funny to see both sides on this topic, where you guys are complaining about ineffective defensive fire, and others are complaining about units firing too many times with defensive (opportunity) fire.
As Mark noted, the latest patches add in optional rules where the defending units do a guaranteed 100% defensive fire prior to melee, so that should address some of your concerns.
Personally I find it very unrealistic for units (in the BG & Manual defensive fire systems) to be able to march across an open field and not take any fire. That would never happen unless they were simply out of range.
I played the BG series for about 5 years before and after the HPS titles came out, but I put them away several years ago and only play the HPS titles now. There's just too many benefits to stick with the old titles.
The PDT files are modifiable, so if you want to change fire factors, stacking limits, movement rates, etc. you can...
And finally, the run down on each of the optional rules can be found on the screen where you select them. Just click the Help button and it will bring up a description of each.
Cheers!