[RPG] say out loud what I ask during a suggestion

The best way to judge what classes should have access to a spell is to compare it to the kind of spells those classes already have.

Wizard and Sorcerer compulsions tend to be forceful in method and drastic in effect: The examples you gave describe subverting the victim's normal mental process by forcibly inserting a foreign thought, and blatantly stealing control of the victim's mind.

Clerical spells, on the other hand, tend to use more straightforward methods of achieving their ends, and often express the role of clerics as spiritual leaders: Thus, the spells that allow them to speak with authority and be obeyed.

As for why the power of Power X isn't compelling... Well, if you look at the ecology of gods as described in, for example, the Planescape setting, belief is the food of the gods - without it, they lose strength and eventually starve to death, or at least enter a state that is for most practical purposes equivalent to death. A follower who only vaguely follows their patron deity's teachings is still more faithful, and therefore more valuable, than an unbeliever magically compelled to serve. Of course, individual gods might have a thing for magical compulsion, but that's not enough for something to get included in the standard priesthood package.

Oh, and like mxyzplk suggests, there's more game design-y reasons for it, too: if Clerics were good at all forms of magic, they'd be far superior to wizards, even enchanters - and playing an enchanter would be much less interesting.

My guess is that merely telling truth might work, as the suggestion could nudge the NPC into Bond Villain mode:
"these people are no threat, so I will reveal to them my great plan!"
"I can totally trust these people with this secret that I have been harbouring all these years!"

It does not ensure that the NPC will actually tell the truth, now. If, say, an innocent was Suggested that he reveal his plans when he actually has none, he might spin some up just to please the suggestion.

Your given wording, though, is not a good suggestion. A better one might be something like "You want to show us how dumb we are and reveal your real plans." Or "You absolutely trust me and will tell me everything you know." Your DM may actually like it, as it gives him a chance to roleplay a monologue.