I'd rather trust what the wise Guinan said.
I just want to know why did Starfleet have the 22nd century Borg incident hushed up until about 2 centuries later? Because I would hold Starfleet personally responsible for the havoc caused by the Borg during the years they were first 'officially' known. That would be an interesting story to look upon.
I imagine it was classified at the highest levels and eventually forgotten entirely. Two centuries is a long time, after all, and even if it wasn't, I can't imagine too many people that would connect the dots between a strange encounter with unnamed aliens trapped in Earth's polar icecap and the nearly indestructible Borg who were first encountered across the galaxy.
Dunno why it would be classified to begin with.
Could make for problems as regards exploration and diplomacy. If all your culture seems to be interested in is looking for killer robots that aren't there, other cultures will probably start looking at you kinda funny.
Also, you may start overlooking more pressing and immediate problems in favour of what was, on the face of it, one isolated incident in a very very big Galaxy. Compared to the Xindi probe that carved a big hole in Florida around the same time, and the Romulan War just a few years down the track, this was very small beer indeed.
There is also the security aspect as regards other groups. OK, maybe the Earth Alliance / UFP couldn't figure out Borgtech and how to deal with it. But, would they REALLY want even their limited data falling into the wrong hands? There would be individuals and groups less constrained by questions of ethics or safety, who would probably have no problems with using / misusing this technology any way they could.
As an extreme worst-case scenario, imagine that in the time between TOS and TNG, the Cardassians (or the Romulans, or the Breen, or whoever) developed and used crude Borgtech-based weaponry against the UFP. Things could get incredibly nasty rather quickly.
I can appreciate why secrets are kept. Not all secrets SHOULD be kept, and I don't believe in secrecy for its own sake. But, it is incredibly easy to invoke 20-20 hindsight and smugly judge how something-or-another should or should not have been done a long time after the fact. Bear in mind that the people stuck with these kinds of decisions in the first place are often acting on limited data and even less time.
Saying that making something secret was the wrong decision - because something BAD happened two hundred-plus years later - is grossly oversimplifying, to put it nicely. IMO