It's not solely about "I only use it for 75% of the fight" it's the fact that it's such an expensive skill, has zero group utility and becomes useless when I need it the most. Let's break down some numbers. On my tooltip it costs 1973 stam to activate. It deals 16910 disease (so it's not buffed my morsg tong) damage after landing five light attacks and spending a further 987 stam. So best case scenario, you activate relentless, which takes one second then proceed to have five light attacks connect and not have to refresh, incap, trap, poison injection or endless hail. That's a minimum of six seconds to deal 16910 damage. Or maximum 2818.333 per second.

And if course you'd be more focused on activating the bow proc after you refreshed the buff from the master's bow poison injection or keeping endless hail up for your enchant or activating trap for more crit damage or activating you incap before bar swapping for extra damage. And that's assuming that frames are crispy and you are in a position to land a light attack.
So realistically you're looking about 7-8 seconds on average which is 2,415.7 to 2,113.7. Perhaps even a longer time and lower damage per second from the ability if there is connection/targeting issues or the player isn't that skilled at light attack weaving.

And that's all it does dpswise. No added buffs no resources gained for slotting. Nothing.

That's not OP by any stretch of the imagination. Lord knows if I could slot something else for a passive I would.