Even so, the rate at which u get amp from Witch is far greater then spearwoman ( even if it can get undo by other curse ).
Greater yes, far greater no. Holy freeze + cursed is pretty reliable in any large pack, often 10-20 monsters clumped for 40-64% per tick, every 2 seconds. Siren doesn't cast amp every time either, so its just another layer, and overwrites amp sometimes.
The real benefit to siren, venom lord, flayer, etc is against isolated bosses like diablo or baal where holy freeze + cursed does almost nothing (no holy freeze procs, just 5% on attacks)
But taintlocks already fry bosses so they don't care. Fire warlocks don't care about amp either, or purge ES, and pure demon summoners get their DPS from spearwoman, so really only subpar blood boil builds skipping tainted, or physical ES warlocks, care about amp on bosses.
technically
any build can prebuff 1 point bind demon to get 15k+ damage on a spearwoman, assuming they stop at 10 demonic mastery, but 20 mastery builds get it to 20k+
for demon builds its an obvious payoff for already using skillers and other +demon gear or magefists or so on.