1. It's a waste of development time and would inevitably introduce more class balance issues, especially in PvP that would waste even further development time, which means less time spent on new content.
I do not recall a single thread here or on Arena Junkies complaining about healer DPS before the PvP power nerf, which cut healer damage by 30% or 40% because they made PvP power only affect heals in PvP.
Balance does not take away from content development time. They have different teams for different parts of the game. Blizzard has said this several times in the past. Luckily, we don't have to worry about this concern.
2. It's 100% unnecessary. Every healing class has a DPS spec, and dual spec exists for a reason.
In PvP, healer damage was an important dynamic which went out the window with the PvP power change. I understand DPS specs will do solo PvE content faster, that's intended.
3. It will be a problem in PvP because healers are not balanced around being able to kill DPS 1 on 1. As soon as you give healers more offense, you risk making them both unkillable and able to kill anyone else.
Healer damage used to be much higher in PvP and I saw no complaints. Healers being hard to kill is intended, and the developers are being very liberal with Battle Fatigue.
4. It will create problems in PvE raids too. Priests and Monks are both capable of contributing a fairly significant amount of DPS while still healing. If you buff that DPS any more, it becomes a class balance problem which makes those healers stronger than healers who don't really have a DPS to heal mechanic.
Then this should be taken into account. But are Discipline Priests really that dangerous smiting enemies for 7k at 90? (I do not have a priest, this is the damage I see in BGs)