Warlocks have a lot of tools, but they're not popular for M+ because the majority of warlocks don't actually use their tools. Other than the genuinely class specific stuff (healthstones and gateway), there's nothing wrong with the lock toolkit - offensive and defensive magic dispels, interrupts, CC, etc. So many locks use imp in dungeons because "the felhunter dies all the time" and "there's three other people in here who can interrupt".
They're not wrong about felhunter dying, but most of them don't do defensive dispels either "the healer can do it".
You're bringing basically just damage, and none of the specs are particularly high damage that would warrant bringing them for damage alone.
If you do use felhunter, now you have access to tools that other classes also bring, and you're probably ressing him a lot hurting your already mediocre damage.
Locks are fine for 15, but I don't see a reason to take a lock to a high key for people pushing .io rather than gear.