"So would you be happy allowing those who were detained under preventative measures to be allowed to carry out an act at a later date, all because the law states that you cannot be held indefinitely for planning?"
Well, there will always be false positives and/or false negatives. And equally, intelligence information is often highly sensitive and I wouldn't argue for all information to be in the public domain immediately. But if people were detained because of a degree of confidence then it should follow that either sufficiently compelling evidence ought to be discoverable, or it should be accepted that maybe the confidence was misplaced.
How many innocent lives equate to the retention of civil liberties, or how much curtailment of liberty is worth zero deaths? It's an unanswerable question at the core of any "free" society.
If a terrorist attack occurs, who do you blame most within your own culture? The agencies who didn't detain them preventatively, or the governments who pissed other countries and cultures off?