It's not about imminent danger. It's about sniping. Druids can go ahead and throw a Rejuv on a target at 80%, but a smart heal ends up topping the player off before most of the HoT duration.
Sounds like you're either over healing or your healing team is just bad. If no one is in danger of imminent death then why snipe the heal? No reason to.
You're assuming people are intentionally snipe healing. This isn't always the case. Healers have smart heals. Most of these smart heals are important parts of their rotation or go-to spells when a large quantity of people need health. If 3 people are at 50% and 1 person is at 80% with a Rejuv already on them, the smart heal is going to hit that person with the HoT on them.
Secondly, let's just assume we're playing with people who don't know how to adjust themselves for a druid. Why is this a valid excuse for us being ineffective at our job in a raid? Another class heals the raid faster than us without any punishment (such as running oom). Are there any other healing classes that require other healers to adjust so they can properly do their job? I can't think of one.
Third, imminent danger is something that needs to be discussed as well, because druids don't have a way to deal with this like other classes. We have 3 aoe heals, and none of them help us in this area which occurs on most boss fights:
#1: Wild Growth - Great spell but it's a HoT. If people are going down quickly, this does't help. We can cast it and pray the other healing classes keep up.
#2: Mushrooms. Clunky, would require being pre-cast, don't heal for enough.
#3: Tranquility: I actually think this spell is good, but it's cooldown is far too large be a go-to. I use this when I see the other healers can't keep up, and usually it's just enough to keep us from wiping (although it's not uncommon for a battle-rez to follow this situation).