What is it’s worth?
10% multiplicative damage debuff on target(s), so relevant for the whole group or even raid. It is the strongest group debuff that exists.
When does it become useful (How much dps can you sacrifies for exposed to still make it worth it)?
In theory, in a group with 3 DPS and a tank, a DPS can sacrifice up to 15% of his total DPS if he can in turn bring a 10s exposed on a 20s cooldown, which will average out 5% extra multiplicative damage for the entire group. In a group with 4 DPS + tank, this number should become 20%. In a raid, any exposed source is more valuable than personal DPS. Of course, this does not matter if, as DPS, you contribute more to the parses than other DPS, so your DPS sacrifice to bring an exposed would be more significant.
And who can cast it even how easily and why would or wouldn’t they do it?
Among the most efficient ways to apply exposed, any pistol user can pretty much solo-expose and debilitate by playing with Kill Blind and Trick Shot. Hammer DDs put a 10s exposed with Eruption. Chaos users can use Reality Fracture (i do it sometimes as sustain-tank if i can free up a slot). As shotgun user, you can solo-expose a target with Bombardment, and it actually becomes a DPS increase over Full Salvo with its passive on targets with big hitboxes such as Ur’Draug (for small targets, you can probably remove Bombardment’s passive and take something else). For the rest of the weapons, it is a matter of special abilities.
Then, whether you “need” an exposed or not is mostly relevant on how much you’d like to bother people to slot something else then their comfort-zone build.
@Onevia Blindside does not expose but purges. You’re mistaking it with Rampage.