The uptime debate comes from both sides. In pure pressure sence slam does more damage than a FR charge. But since in pvp you often swim in rage its rarely an issue. But there are a lot of cases where slamming would be better than stacking FR.
This is a conflict that fucks me up personally tbh. (Fuck my rotation OCD or whatever you want to call it.) I wish FR just replaced slam and was balanced accordingly. Having two fillers is just annoying.
Totally worth it vs casters or combos that dont stack. But vs melee combos you might prefere to pick up sweeping + combo.
Focussed rage is viable in pvp. But it's easy to mess it up in the heat of battle. (wasting charges or draining yourself out of rage).
+ it's nerfed in pvp with 15% each stack. It's great for burst but meh without.
To a poster above. Mortal combo does indeed give you two mortal strikes. When tactician procs your second charge keeps cooling down, meaning you effectively reduce the avg cd of ms.
Example: Tactician procs of ms> cs> ms = 2 gcds. Meanwhile your second ms charge has been recharging within those 2 gcds. Which means you can ms sooner than you would with only 1 charge.
Just check your UI or any addon that tracks it. You see he second charge cooling down on the background while your current ms keeps benefitting from tactician procs.