Possession stats aren't really the best thing to use to judge someones defensive ability. Obviously they are important but if you are the type of defenseman that is good positionally and with your stick but not a great skater or puck mover you are almost sure to have poor possession stats since it's more difficult to clear the zone. That doesn't mean that player isn't good defensively though. If he's always in the shooting/passing lanes, able to block shots/passes, able to keep guys to the outside and prevent them from driving to the net that's the type of player that plays well defensively but won't have good possession numbers. There are different types of possession. If you're a good puck mover like maybe a Del Zotto sure you'll be in your own zone less but when you are in your zone you're getting beat 1 on 1 all the time, letting guys around you, giving up point blank shots. If you're less good at moving the puck/skating you'll be in your own zone more of course but the quality of chances will be lower.
Blindly stating that possession numbers have a 100% correlation and are the number one determinant of player ability is a misuse of these stats. Imagine the extreme case where player A is in his own zone 50% of the time and never blocks a shot and player B is in his own zone 75% of the time and blocks 33% of shots (and they get out of the zone). In terms of chances allowed defensively these are equivalent situations but just looking at one number of possession you would state A>>>>>B
