Do you need to use exposure and for example "penetrates X lightning resistance" vs all mobs?

Hi, noob question. Running some strange fire/lightning sorc own build. Mostly running t15 maps, and im not sure if its worth using skillpoints as example Breath of lightning (penetrates lightning res= , or just skip it and go more +dmg.

If i encounter a rare or boss with for example lightning res, i just curse them witn -35res and use another skill to put exposure on them.

So the question is, do all mobs in t15 have resistance to fire/ligtning/cold. Is it worth spending skillpoints on that, or just put curse/exposure on the specific blue/yellow/bosses with res?

Once you have enough* increased damage and sufficient* speed, some penetration is always worth it. Ok, assuming it's close enough to the path.
It will at least probably have the same effect as a damage node with twice the number at that point.
If you manage with just a curse, and it works, then it works. There's no telling what will work a few patches down the road.
Curse + Exposure is a significant amount of resistance shredding already, though - if you manage to do that without a lot of time expenditure, you're probably ok.

*this depends very much on what you consider enough. Once we know all the mob's stats, you can do the math.

Now im deceding if 60 increased damage is worth it over 27 lightning penetration on mobs. Its hard to tell bec everyting exept rare is getting more or less one-shotted however i do, its the bosses that takes some time. Maybe il stay on damage and curse the bosses + put exposure on them manually. I can ofc also use support gems on spells like fireball that im using, but il go down like 20% dmg vs exposure gem or penetration gem.

Ty for coming back! :-)

If you have to add exposure manually, you can ask yourself:
How much time on the boss could you spend with offensive casting that you're instead using for applying exposure, and how much damage (easy to calculate as you can know all your modifiers) does it cost you to get an equivalent amount of penetration?