is there any reason to use "spinning slash" combo on drk?

Your emnity combo is absolutely necessary, although you won't use it nearly as much as your Soul Eater combo.

In dungeon pulls, you will likely not be using it since you will be relying on your AoE enmity generators such as Unleashed or preferably Abyssal Drain since you are dealing with and maintaining agro on multiple enemies.

Your emnity combo will mainly see usage when starting boss fights or maybe if you need to pick up and establish strong agro on an add that pops during a boss fight when you are the off-tank.

Basically you will normally want to get off at least one emnity combo when starting a boss fight, two if you want to be extra careful or are dropping Grit after establishing initial agro or you could use a DA Powerslash combo if you don't want to spend the extra time with two full combos or want to try starting off the fight out of Grit.

Also if you notice that some of your party members are getting close to overtaking your agro lead on the boss you will probably want to cycle in an emnity combo to regain your lead.

Basically you should not be using Unleash or Abyssal Drain for establishing agro on single targets like bosses (same goes for PLD and Flash, etc), use your agro combo as it will generate more agro and do more damage to that single target.

2x cross-hotbars is enough for most jobs to fit all job actions and role actions. Recommended to keep all your actions set to hotbars and get used to switching bars during battle.

You'll probably have situations eventually where the enmity from power slash combo is needed at least occasionally. Good point though how enmity combos that don't do anything else isn't very exciting design. It would be interesting to see a tank job that isn't structured that way.

Your enmity combo is an MP free hate generation tool. Keep that in mind OP, when gauging its usefulness. Another way to look at it is your tool to lock down hate. Once it's executed on a mob, especially with grit + dark arts, you're not losing that mob ever.

I assure you, the situations where it is more optimal to use your aggro combo will present themselves the more you tank.

If you're going to skimp out on using Spinning Slash, then you'd be force to burn Dark Arts on Power Slash, and that's a wasted Dark Arts as far as I'm concerned. Don't skip spinning slash in your aggro combo.

Spinning slash + Power slash combo is near mandatory if you intend to hold hate on bosses, whether they're in dungeons or primals/raids.

Unleash does generate a nice bit of enmity for crowd control, but without the power slash combo, you will lose aggro ultra fast against any dps that knows what they're doing if you attempt to rely on unleash and unmend for enmity on single target. Not to mention using unleash for enmity on a single target, or spamming unmend for aggro would be a gargantuan dps loss.

I also play on controller, and I've never needed to worry about space on any job. Stick the power slash combo on a second hotbar or the extended/expanded hotbars, as you'll be making your tanking life harder without it if you come across any dps that even remotely knows what they're doing, or a healer that likes to aoe nuke heal.

Spinning + Power (2 GCD) will give you much more single target enmity than 2x Unleash and without using mana. They will also do more damage.

I only do it once or twice on pull and that it.
In case I do à boss with adds like os3 I try to prepare the last step with DA to get a strong lead on aggro on the white wisp.
But yeah beside pull its not super useful, I play on ps4 and it's on a double L2 cross with other things I rarely click (grit, sol survivor for instance)

Power slash is a great combo for what it is. There is absolutely no wind up and it's a huge dose of aggro in one shot. If your co-tank is a PLD, it is far far far more preferable for you to pull first and only spend a combo in grit. If you use Dark Arts, you're ensuring a huge lead to comfortably dps outside of grit.

It is ideal for WAR to pull first, but only under certain conditions like with a NIN. Otherwise your WAR might be looking at having to use unchained, and even then spend a whole combo before reaching butcher's block, and that's very much felt in areas like the start of o4s, where healers can rip aggro before aero 3 casts.

However, DRK has no need to set up prior buffs, has nothing to lose except a combo, and makes life easy. This isn't to say that DRK pulling first is the way, but that it's never a terrible idea for DRK to pull first BECAUSE of how potent and DA PS is to do its job.