The situation in that CoC campaign was a strange and tough one. It was a 1920s campaign and some mob boss had gotten into the Mythos and was using the Mythos beings to do his dirty work for him. Not the really powerful baddies, but still Mythos nasties. It's hard to pin a murder on the guy if the DA dies under mysterious circumstances and the witnesses are either incoherent or babbling on about flying nightmares. For example, just try to prove that a Treasury agent was murdered by Deep Ones because he was disrupting Capone's booze shipments from Canada. It just worked out that it was actually safer for us to try and call up a Mythos horror that it would haven been for the group to get at the mob boss any other way.
Changing stats, and/or keeping them secret if fine. But...
Unleashing a monster on a group of PCs isn't fair play either. Especially if the monster is immune to the player's attacks or is so powerful than it will waste the group or a good portion of it regardless. In D&D, with escalating hit points, and encounters that are "balanced" (read fixed),a given monster can only be so much of a threat. In BRP with Dragons doing enough damage to kill most PCs with a single hit, regardless of armor or parring, it's a whole different game.
One of the reasons (not the only one, but a major valid one) why players often memeorize the monster stats and weaknesses is becuase the way many D&D adventuers and ZGMs just spring things on the PCs with no warning or clue. Let's face it, wih the way the monsters are taken out of context and culture, the odds of a adventuerer finding a monster's wekness is practically nil if he is from a different culture.
What facts the PCs might "know", could easily be wrong. The Vampire's vulnerability to sunlight and the werewolf's vulnerability to silver are both inventions of the cinema, and either could prove to be a disastrous revelation in a RPG. Especially one that is an unforgiving as BRP.