You're counting out that Warrior is auto attacking during all this, that they have dps buffs they can use, that they have vengence adding in more damage, that holy has a long cast time, and above all else, that at most you're losing one or two holys if the WAR is going heavy slaying.
You're talking as though that a slaying warrior means no WHM DPS. No, it just means a small handful of fewer casts while the warrior continues DPSing.