4) The capstone is odd. It gives you a spellpower *opposite* of the path you chose, plus Word of Balance as a spell. (Yay WoB!). However, if you have been leveling a cleric using nothing but light spells, what is the point of the extra negative spellpower? Kind of odd to me.
It's true that the principle bonuses for most characters are probably not the spells from the other side and the +5 spell power, but even taking all of that out we feel there's still a lot of power and usefulness left in the capstone.
There is also the option to choose only Universal Spellpower in the tiered enhancements, if you want to try mixing both Light and Dark.
All that said, the added spells are not likely to be completely useless for the level 20 Clerics, though we don't expect many characters to necessarily try to make full use of all of them. (It's still another possible build option, if a trickier one!)