I've come back from a break, done gk/other with a bunch of newer people, and I think I've seen one person running battle chemistry. At the same time, I've seen many people running bard and other support type classes. Bard sucks (<3 Coglin), but that isn't the point of this thread. I want to highlight what makes battle chemistry a requirement for the best parties when we have a new dungeon on the way.
Quick history: BC used to be super OP. Mobs had lower hp and AOE attacks weren't reduced vs large groups of mobs. Additionally, freezing mist was a 15s aoe root. In time, mobs received more hp, its ability to mow down 10+ mobs was diminished, and Citan even had the gall to hit bc while it was down and get rid of the root (it's now an aoe gravity). While a hollow shell of its former self (two years ago), it still has a bunch of useful/unique tools.
Extra Skin, or just "skin", is the single best support ability in the game. No single move has the capability of guaranteeing a smooth run as much as skin. This ability absolutely shines against the ranged orcs in GK, who do piercing + a poison dot (indirect). When you're in a zone where mobs are hitting for 200-300, 10-20% mitigation allows you to survive an extra hit or two and non-elites become trivial. Despite taking the nerf bat to the face for a couple of years, this skill alone makes it worthwhile/necessary to have at least one bc in your groups.
Take note that you can skin everyone in one gear set, then change to your combat bc set and everyone retains the skin mods. One of the few situations where gear swapping is encouraged in this game.
Let's say your puller messes up and gets 4-5 when you meant to get 1-2. You guys might get away, but one person is likely to die. Have a BC and your group on discord, pop haste concoction and run away in unison. Nobody dies. If you throw up freezing mist to slow them then haste away, it's a built in "get out of jail free" card. It might only be used once per run, but that's a lot of trouble you can avoid by having a bc in your party. Plus, it allows your puller to be greedy, optimizing your play, knowing that they can just say "oops" and run away.
300 health/armor and 80 power every 15~30 seconds (your nukes have a mod that decreases healing mist recast). There are a few other aoe heals, this is still a very good one though.
Your nukes are "meh". They all hit for 1400-1600 in an aoe. The addition of burst evasion has REALLY hurt bc (srsly though, how do you miss with an aoe bomb?). You tend to see people pair it up with strong other classes, fire/bc is a classic aoe farmer, priest/bc can heal, archery/bc was once very good but is kinda unfortunate now. Ice/bc for cc (creep/crowd control) is maybe worthwhile now that freezing mist (bc aoe gravity/slow) and tundra spikes (ice, aoe root) don't build resistance to each other.
tldr: Skin op, level bc
Edit: I forgot to mention that skin is more powerful than it was 6 months ago because crits no longer increase damage done. The more consistent damage is, the better that is for skin.