Oh yeah. Parallel Construction has been around for a bit. I work with some LEO (local, state and some feds) who used similar "tactics." I hate it. I hate it so goddamned much, but I keep my mouth shut. I have a few reasons now.... but once the "now" runs out, we'll see.
No one (I guess there's never an absolute, but you get my drift I'm sure) can use parallel construction for a case. However, parallel construction is just more information about a specific individual(s). Then LEO waits until that person does something illegal, or they find out they did something illegal in their past. Maybe a warrant from a different county, late / non-existent or months late child support payment, etc.
Like you said, PC is mainly used to ID a person. Once ID'd LEO follows the suspect around until the suspect fucks up, or they have something in their past, and then bam, that's it. Then "oh, yeah we arrested you for A, but we found more information about Z when we were looking at A, so now you're going to jail for 10 years instead of 2 months..."
I've seen detectives go through every possible means to try to link the suspect (no one knew who the suspect was yet, but they had several people that it could be) to a case. Once a detective was able to obtain information about several different people from some agency (they don't tell me who gives the info yet), then they could start looking for anything to just get the suspect to talk to the police. Maybe their taillight is out and the officer is just giving them a friendly warning, anything to get communication going with the suspect. Then they can start building a better profile of how the suspect might act. So many different options (legal and illegal) open up once you have names.
Anyway, now they knew who the person was but they still couldn't do anything until the detectives / LEO found something illegal that the suspect did. The information had all the pieces of the puzzle the detectives needed to identify the person, but nothing to put the puzzle together. That is LEO's / detectives job. From the cases I've seen it is usually a warrant for their arrest from a surrounding county / state (for not paying a ticket, or something small like that), late child support payments, invalid vehicle registration, etc. Anything that wasn't worth chasing around for them before, now was worth it because they had more information regarding Z.
I'm going to be honest, I spent like an hour writing a reply to you. In the end, I have a feeling we're on the exact same page and it was just going to be a bunch of text agreeing with statement. I'm also very passionate about these types of things for some reason, so I just kind of blabber on.
In the end, every step of the system needs reworking badly. Local, State, Federal, everything. Before I took this job, I had an idea of how bad things were at each level, but this opened up my eyes way too much, and now I can't close them anymore.
Also, I'm not LEO, or maybe I am! Regardless, I just earned their respect after many MANY years. I also do have to work VERY VERY closely with them for my job. They NEED to be able to trust me, as much as I NEED to trust them. So we keep tabs on each other for a while. I found the information they have on me, and the detective I was working with at the time. This agency didn't black out anything for anyone working there either. Some agencies do I'm sure, so my experience is probably (hopefully not!) in the minority. The big thing I learned from that is, everyone does something illegal. There's far too many laws that there's no way anyone can know everything about them.
It fascinates me to see both sides of the fence. However, (prepare for a debby downer deal) it does begin to destroy your mind and soul. Drinking, drugs, hobbies, nothing I found can even dull what I've seen from just helping those people out. I can't imagine the people who have to actually deal with those fuckheads. They deserve more money than they get. The ones who do things legally of course. The illegal ones are a different topic...
Good chattin! I always enjoy a nice reddit chat. Even if it strays off-topic a bit. If it's all over the place, it's because I just woke up so excuse that.