'Cause even in 5-man you could find urself in the situation in which you wanna cast another AoE heal beside CoH. CoP is more mana efficient compared to PoH in any case (not to mention it is more than Halo too) and benefits from Serendipity buff.
Actually you do 'cause Disc has no another cheap AoE heal.HN is useless unless there are like 10+ people stacked, which also won't happen in most circumstances. Obviously, you don't even need this on the action bar in 5mans.
Not really, 'cause you cannot know when exactly you will need that extra output in your AoE heal. What if you need it NOW and you still do not have enough stacks? You throw around random heals to reach 10?[WoM] Still much better than CoP for 5mans.