You're paying for weapon upgrade. You get this ability as a bonus. It comes at 3CP because of the weapon upgrade.
You have chance to use it after each engagement. If you don't do it it's your own fault.
It shows EVERYTHING on the map for couple of seconds. You'll be able to gain this Intel after next fight as there are always some wounded models on the ground. Again if you don't it's your own fault.
Lastely I can tell when my opponent is going without Radio intercept based on the map I can roughly say what's going to build seeing his initial units composition as well.
I didn't say G43 interrogation is bad, but they're honestly not in the same field as far as intel goes.
Radio intercept works at 00:00. It works while there's no engagement going on. It works if you lose an engagement / are driven back and don't get a chance to use interrogation. It works if a G43 model doesn't happen to be around (had to retreat, on a different front, whatever). It works all the time instead of periodically.
For that matter, G43 interrogate only reliably works when you have decisively won a skirmish. You're not going to rush your G43s out of cover just because one conscript model went down without risking them getting killed or otherwise taken out of the fight. It works if you catch a lone squad way out of position and get lucky that the model gets wounded instead of dying, but that's about it.
I play about 50/50 sov/ost and 1v1/2v2 and I can count the number of interrogations I've pulled off (a bit of hyperbole over so many games).
EDIT: To be clear, I think both are kind of lame abilities that remove a lot of reaction, shock, and planning. Just that radio intercept is much worse. G43 would be fine with just having the interrogation removed, it's a good enough upgrade by itself.