If your average dps is 1000 on a fight you will lose 1.5 seconds refreshing the Crusade trinket with a nonsense spell. That is 1500 damage lost. To make up for the damage lost you will have to do straight up damage for a long time (hard to calculate because you would be building up the stack again if you decided not to cast the nonsense spell). Anyway, it's usually NOT worth it dps-wise to "save" the stack if you can cast a damage spell instead. Being affliction and having had the trinket for a while now:
- Maulgar: no, unless you're on enslave duty.
- Gruul: yes, Shatter makes you lose it usually.
- Magtheridon: Yes, cube duty.
- Hydross: Yes, banish duty, need to stop dots at transitions (some guilds)
- Lurker: yes. the submerge time until adds being targettable makes you lose it.
- Leotheras: depends on how far away he runs.
- Morogrim: yes, water tombs
- FLK: no
- Vashj: yes, but only phase 1->2 unless you're very lucky with elemental spawns
- Al'ar: yes, usually every meteor
- Solarian: no
- VR: no
- Kael'thas: yes, phase 1, MC, Gravity Lapse
- Rage: no
- Anetheron: yes sleep
- Kaz'rogal: no
- Azgalor: yes, silence
- Archimonde: all the damn time

- Naj'entus: yes, shield
- Supremus: usually no
- Shade of Akama: no
- Teron: no
- Gurtogg: no
- RoS: just between phases
- Shahraz: yes, FA