This sounds like "Stances", which means you are locked into either heal or damage mode for a set time. It's been mentioned a number of times over the years.
And my answer is always the same.
As a druid, my primary playstyle, as well as one of the core advantages / class defining abilities is flexibility. You can heal and you can damage, you can be
a floor wax and a dessert topping.
So, within a minute, I can throw some heals, barely saving another character's life, throw some dps on some mobs; and be flexible to do either as the situation demands, especially in very tough rapidly changing events, like raids.
Locking me into one or the other means I somehow have to both predict the future (whether during that "locked" time I need to be healing or damaging) as well as not letting me use one of the druid's primary strengths, the ability to fill in what is needed at the moment.
The AA Spirit Wolf forms used to be "white wolf" (healing) or "black wolf" (damaging). It was, basically, a stance much like you are saying, only I think it just provided a bonus to healing or damaging and didn't have a negative to the other. Still, it had the same problem, it was in opposition to the core of druid play style.
Thankfully, those druid AAs were changed to "Spirit of the Great Wolf" (one group and one self) which improved both direct damage and instant duration healing spells, fixing that conundrum.
To "sacrifice" one for the other is a step back to a design idea that was discarded due to the reasons I stated above and,
I believe, the change made, leading to the current improvement of both healing and damage, was greatly appreciated by the vast majority of druids.