I was reading a thread on the WoW forums about what to look for in a healer app. The thread devolved into arguments about what is acceptable in a healer's User Interface.
Personally, I think that as long as you are comfortable with your UI and are effective with it, then all is good. I personally use Grid and the default UI. Mostly because I find that too many add-ons cause performance issues and unforseen bugs, and make patch days miserable.
However, here are my three criteria for evaluating healer UIs:
1. Can the healer see the health bars of the entire raid?
It seems pretty obvious, but back in the day there were a fair number of healers who only healed their group. Less common now that there is a decent default raid UI.
2. Can the healer see all relevant buffs and debuffs?
You must be able to see all the important buffs and debuffs in a fight. Ideally, your UI would allow you to filter the debuffs, so that you don't see the unimportant debuffs.
This is one of the major reasons I like Grid, because I really like its debuff filtering capabilities. Being able to get identify at a glance the people with Chimeron's Low Health debuff or Explosive Cinders on Heroic Nef makes healing those fights a lot easier.
3. Can the healer see the fight?
It's really easy as a healer to "tunnel vision" on your health bars. Sometimes your UI makes it even easier, and makes it harder to view the fight. But you have to be able to keep an eye on what's going around. A good healer UI, somewhat paradoxically, needs to emphasize the fight rather than the health bars.
The nature of healing drives the healer to focus on the health bars, and in some ways the UI should pull them in the other direction.
So those are my three criteria for judging healer UIs. But again, comfort and effectiveness are key.
However, I believe that all healers should be able to use the default UI if necessary. Your favorite addon could break, or stop being maintained, or cause you to crash on a specific fight. You should be able to at least be somewhat effective if you had to heal without third-party addons.