The problem is the relatively long CD of pet abilities vs. the abilities you mentioned (Sunder/faerie fire). The warrior/druid is always going to have higher uptime, even on switched-to adds, because they have 0 CD and a small resource cost to cast these abilities. Same with CoE from warlocks, same with physical damage from DKs. That's why I far prefer buff pets to debuff pets.
The advantage in the case of the tallstrider is that the pet applies 3 stacks at once as an AoE. The CD is rather long (25 sec) but if you are tanking a bunch of adds together and you have other buffs covered, it's definitely the best option. Also, if you have a warrior or druid applying weaken armor on the primary target (we don't) you could just manually cast the armor debuff when there are adds so the pet doesn't spend cds on the boss. There is no class which can apply weaken armor that can apply it as an AoE that I know of, and applying it to all the adds manually is usually not a good use of their resources (depending on how many there are). Wind Lord is definitely the perfect example of a tallstrider fight, but it can also be useful in other situations, like Stone Guard and Elegon.