Idealy the bullet should penetrate all the way through the deer but expend 99.9% of it's energy in the vitals. Then it should just poke out the other side and fall to the ground. That would be the optimum performance.

That is seldom how things work out. There are too many variables to account for when hunting. So you pick the best bullet and load you can to compensate for the possibilities and you go from there. Most of the time you are better off with more penetration than with less. Much better to pass through the deer and take out the vitals than to have the bullet blow up inside the near side and not damage enough of the vitals. The bronze-tipped bullets of the 50's had a nasty reputation for doing that. The added assurance you get with a better bullet means even if you hit a bone or have a quartering toward or away shot, you can still get enough penetration to reach and damage the vitals.