CH and PoH heal for ~1100% with a 2.5s cast time. HN used to heal for 562.5% with a 1.5s GCD. Then consider that PoH costs 7.1% mana versus HN's 1.6%. How can you possibly justify giving HN a HPET that's 85% of PoH's, but yet costs only 22.5% of what PoH does?
This wouldn't be an issue if HPS were the only concern, but efficiency does matter in WoD too. Allowing Disc access to such a mana efficient heal, that I'm also pretty sure is unmatched by any other non-CD heal, is poor balance as well.