I agree. Protectra 5 is just a laughable 5 def more than Protectra 4, and even with full Protectra 5 merits, the increase in defence from 4 to 5 is smaller than from 3 to 4. In a game where defence already makes a very little impact at large values, this spell isn't really worth the single merit point you need to unlock it.

For Protectra 5 to be anything close to worth spending a merit on, the def bonus should probably start at 20 point more than protectra 4, and each merit probably add 3 def to that.

I do however also think that changing the protectra and shellra 5 merits into protect/shell potency merits, so that they will affect all protshell versions, including single target versions.