People have their own reasons for playing a class and it varies from person to person, some think a class looks cool or feels nice to play and so they will play that of course.
For me, I love the combat flow of RDM and MNK in PvP, so I like to play them often, though fortunately for us in PVP we can swap to any class to try out so there's not really any restrictions to what a person picks.
As I said before, it's an easy solution to use marks as they will save your life, If you're running in and blindly tossing macrocosmos into a clump of players every 30s with no consideration for your own safety then it's something you need to account for yourself if you want to avoid 'randomly getting oneshot' by a cheeky LB.
The LB combo is lethal to any healer that often pairup with the DRKS, the damage is lethal and you brought up not being able to 1-shot squishies. If the DRK is diving directly ontop of you in particular and you're a ranged class getting pulled, then you're considered out of position and you have been caught.
Yes they do, that's why they often have incredibly high win rates compared to random players. People often 'lead' alliances and direct them to objectives, or have things like macros counting down when to coordinate burst for groups. And as you described, they engage HARD, sweep up any leftovers and swiftly leave to the next node/point worth contending.
BH4, but you're basically given BH5 for free because you can do the Drill+LB from BH3 on squishies to secure a kill every 90s ontop of any other assists/kills obtained in skirmishes. Also it's possible to use Wildfire with single shot before using drill+LB for an extra 12k potency ontop of the 58k which becomes overkill at that BH against a ranged target, but it's good against the melee players since the built in damage reduction makes them quite durable compared to ranged classes.
I don't have much else to add going back on topic as LeoBertolaso's post summarises any extra thoughts I may have had regarding Samurai's uses in FL, and yes it's worth giving a go to try at the very least a few times to at least see how it operates.