Defense bonuses look a little runty taken on their own. They start to shine when you combine them with other things, particularly HP recovery mechanics, or offense/speed reductions for your enemies.
Quick example:
Let's say you have 100HP and an enemy is hitting you for 20HP a turn. Currently, you will go down in 5 hits.
We'll give you a modest defense boost, say, just Protect's +25%. At +25%, the enemy's damage per hit will drop from 20 to 16. It now takes 7 hits to kill you: if you ask me, this is already a pretty decent gain for just slapping a buff on.
But let's go further and slap Weaken on the target: -20% offense. Now that 16 is being cut to 12.8, and it will take 8 hits to kill you.
Let's toss a Regen effect on you for +3 HP/turn. That effectively cuts directly out of the 12.8, dropping it to 9.8... now you need 11 hits to drop.
With 3 actions (Protect, Weaken, Regen) you've more than doubled how hard the target in this scenario is to take out. Worth noting is that further healing will drastically improve the effect further: a 15 HP heal would barely be worth mentioning on the original 100/20 example, not even buying you an entire hit. But once it's at 100/9.8, that 15 HP buys you nearly two more hits.