Or is it just me?

It seems like specing into utility actually does more to increase your survivability by making you more mobile with quickness, summoner spell reduction, CD reduction, keeping your mana and health up with improvements to regen, more xp so you level faster.

Am I missing something here?

All the defensive tree provides that is valuable is strength of spirit and veteran's scars. The rest of the tree is a load of **** because all it does is make you more of a rock that can get beat on, when going with utility might get you out of 1 more hit by making you more mobile rather than being able to take 1 more hit by using defense and raising your effective health by a few hundred points.

Also, if I'm tanking, I'm probably getting ignored as much as possible in team fights, so wouldn't it make more sense to spec into offense or utility to make yourself a better carry harrasser?

In my eyes, 9-0-21 is by far the most useful spec followed by 21-0-9 which is more useful for physical carries.

ADDED:

having havoc increase your damage by 5% is pretty worth it for alot of champions.

And talking about Tenacity (the 21 point end of the defensive tree perk), why would you want this if you're the tank? Are you a math n00b? you'll get your armor up to a point where you're probably blocking 60-70% of physical damage, so you're talking about reducing physical damage taken by **4% OF THE 40-30% remaining damage that you take, so only about 1.75% of the true damage is reduced on attacks as the tank with this perk... mega useless..

Discuss

deff not. the extra dodge is amazing, and the 21 defense points for melee champs (not ranged) really really helps

Being more of a rock is the point of being a tank. I don't need these.. Utility **** points while I'm tanking. I hope you're never my tank.

It's almost impossible to get 21 utility on non mana heroes. I personally go 9/21/0 on nonmana heroes that are not crit/AD based (Akali and Vlad to name a few).

If the hero uses mana, I will go 21 utility by default. Once I get to know the hero I might switch it up but x/x/21 is such a good spec for anyone who uses mana, I agree with you there.

Some champs benefit greatly from defence masteries, like Jax and the dodge ones. However, on the whole, I'd say it was the least used of the three. Could use some adjustments.

extra dodge, 70 hp, useless? how?

Offensive tree is the useless one.

Only good talents in that one are the T1 talents CDR 15%mp and armor pen.

The crit damage talent is too far down and situational for most. Only champion I'd say its 100% needed is trynd. Edit: Critplank too.