Having had my share of good and bad blood trails(defined as lot's of blood and little to no blood) with both gun and bow and helping others with their trailing I feel I can come to a few conclusions.
I think the big issue you are trying to bring up, Mike, is whether an exit wound is neccessarily better for recovering hit game and whether it would be better to take a "less lethal"/"slower acting" shot in hopes of producing a better blood trail to follow up.
I don't think there is any question that two holes will leak more blood than one and that two holes should be desired. However, the placement of these two holes is critical in the amount of blood spoor produced in my experience.
Broadside double-lung complete penetration does not guarantee good blood trails. Generally speaking, higher holes leak less than lower ones. This is one advantage to achieving an exit hole when shooting from a treestand...a low exit hole to drain blood from.
Take a ground level broadside shot for instance. Whether it is a high lung shot or low lung makes a difference. Internal bleeding(hemmhoraging) will occur in both of these instances and the life expectancy of the animal would be comparable, yet more spoor will be left for the hunter on the lower shot. I find it more desireable to shoot lower than have an exit hole neccessarily.
Much of the discussion thus far has dealt with whitetail deer and elk who's vital organs are fairly accessible. Broadside behind the shoulder shots are efficient means of dispatching them. However, when we consider game such as hogs and many African species who's vitals are placed further forward in the chest, exit holes are not always expected. In these cases marginal shots taken to ensure exit wounds would be irresponsible, in my opinion. It would be better to get a broadhead in the vitals with only one entry for the quick death of the animal. Would you rather risk missing the vitals by aiming a bit further back, or aim right in them whether you get an exit or not?
Another thing to consider. Deer blood coagulates awefully quickly clogging holes on not-so-expired animals. This brings to question the role of an arrow staying in the animal vs. complete pass through. I can tell you from experience that no deer I have seen shot with the arrow still in it has stayed calm. They take off like a bat outta %$#@! This most certainly will lengthen trails. An arrow still in the animal without an exit may be doing some slicing especially when running through brush, etc. However, consider this: If you get stabbed in the leg with a knife you aren't supposed to pull it out. If you do, you will bleed much more than if you left it in and got attention at the ER. It could mean life or death. Why is that? Blood coagulation, natures mechanism to stop cuts from killing us! The knife is aiding this process by lessening the degree to which the blood will need to coagulate to stop the bleeding. Now, I understand that deer don't remain still(far from it) till the arrow is taken out, but this removal will result in more external bleeding. An arrow could be doing the same in both entry and exit wounds if left there.
We as hunters try to minimize the variability by taking "high percentage" shots and using razor-sharp broadhead tipped tuned arrows out of proper bows, yet that variability is still there. That animal may move, that cut path could swirve in the animal, a rib might be hit. Frankly, that is part of what makes hunting so much fun...the unexpected. Good topic Mike! Matt