When a multiplayer game is made, players will try to beat their opponents down while not getting killed themselves. Inevitably, anything that is a one-hit-kill, effective or otherwise, becomes popular for the sole reason that it means that the opponents can never make any mistakes. Now I can point out why this is hypocritical because of their annoyance about single player Demonic Spiders, but I won't because that would be too cheesy.
That is appropriate, because defensive power is designed to stop cheesing. By being able to survive attacks well enough, you can fight back and punish the opponent. This is useful for some good tactics.
First is being able to attack strong defensive positions. Being able to survive hits allows you to get gradually closer to where you want to get, and being able to boot out anybody who can't rival you in power well enough means that staying in one place for too long will get punished. This alone is useful for making defensive power useful, as it stops camping from working as well, allowing for more offense tactics to work and ultimately give the game a better sense of fun as a welcome side effect.
That's not even the only useful tactic. Being more sturdy also means that you can counterattack better, and potentially force the opponent to think better, if not outright scare them off. And if they can meta-counter you, you can actually live through it and adapt. Also, some abilities can get triggered by getting hit, and having more room for choosing when you get hit can easily make them VERY useful.
In addition, as a welcome indirect result, some other traits become useful and therefore easier to balance. Range would be less polarizing by providing an acceptable weakness, so Frail Snipers can be made more accessible to newbie players without making them too powerful to deal with rather than making them simply Mighty Glaciers that end up with hit-or-miss balance; attack power characters are actually useful because they'd punch through the Stone Wall's defenses; and even The Mario benefits, as he can use both of those traits AND his own defensive power to his advantage.
The only character type that would suffer would be the Fragile Speedster, but a character type like that tends to be overpowered enough, due to the inherent versatility of their speed advantage. They need to be plausible to trap, not too easily but definitely make sure that they have to actually work in order to win.
Now what would result when defensive power actually works? First off, individual strokes of luck will not be as brutal, as more working traits allow for fending them off, and more skirmishing reduces their impact too. Second, the heightened amount of skirmishing in a given match means that a victory results from being consistently better, rather than from managing a lucky strike, but at the same time it's easier to identify mistakes, which results in better metagame evolution, or at the very least better identification of balance issues.
Even if the game doesn't manage to be perfectly balanced, I can be sure that useful defensive power, handled correctly, would be a step in the right direction.