The Defense skill point works the same way as attack except that it doesn't seem to have an effect on damage other than as a point of comparison to your opponent's health.

Now since property robberies are limited by your level as far as the number of mafia that can be brought, having extra defense points will help defend your properties assuming that your mafia size is at least equal to your level. Once again though, at higher levels this becomes way more meaningful, as in my case. As of this writing, I’m at Level 61 and I have 80 for Defense.

Trust me, if you spend substantial amount of skill points for Defense. It’ll be of great help to repel attacks. When I stopped playing in a day, I’m pretty sure other Mafia players can’t beat me easily because of my huge Defense point.