Like would you only use the ability if the demon/devil/angel/etc’s enemy doesn’t know they are there? Because otherwise it seems dangerous to waste such an action with a large chance of failure.
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When the PCs are on the other side of the door, noisily casting prebuffs. Remember, verbal components require speaking in a strong voice.
If combat has already started, because summons have such a long casting time, I would only do it if I have plenty of mooks to delay the PCs and I have some cover so they won't shoot me.
The implication seems to be they only use it in emergencies. However, as some of the most powerful kinds (such as balors) can summon any level 19 or less demon with 100 percent success rate, I would think it wise to summon something any time they face a real threat (unless they already used it up for the day.)