Added 8 minutes later
First, there also needs to be a chance that we will actually find this enchantment. If a Dwarf has all the runes, the Necropolis will have to work hard, and remember, we have Eternal Light + a rune that removes all negative spells.
Yes, and who really needs Eternal Light when you have two main mass spells - Plague and Slow? Eternal Light doesn't affect either of them. Besides, the rune can only be used once, and for the second use, you either have to spend a lot of resources or restore the rune - which takes at least one hero turn. This raises the question of how effective runes really are. The only rune that I think is quite effective is the rune of battle rage (the one where several nearby enemies are attacked without retaliation). The rest can go straight to the trash.