Yepes, to an extent you are correct. Players in an RPG can, and do, do things that no movie heroes end up doing.
Simply put, though, the point is to not just reward the players but to make the adventure challenging, interesting, and fun.
As an example:
** Warning - Minor TGS spoiler! **
In TGS my group (of 3) are heading to the goblin-infested farm. They come up with a plan and make molotov cocktails, which they then light and use to bombard the farm from outside of the pallisade. Most especially, figuring that the shaman they saw was housed in the farmhouse itself, they specifically targeted it with the majority of their "bombs". After all, it had a nice convenient THATCH roof.
Now, I applauded the PCs for their ingenuity. I could have just had the farmhouse burn down, killing everything inside. Really, it what technically/realistically should have happened. Now, that would be a bit of a letdown, not just for me but for the group. There would be no tension or drama, no story or challenge, if the shaman died so quickly and easily.
So, instead, the shaman blasted his way out (blowing out a wall) as the building collapsed.
Yes, you need to be careful that you don't make the PCs feel like their creative (and highly disruptive) ideas are useless. However, I feel that the story/plot of the adventure needs to be maintained so as to remain enjoyable. If the PCs are able to kill off the big boss with a single lucky shot during his pre-fight speech, is that really fun for anyone? You could just say, "Ok, the boss is dead. The enemies scatter, and the world is saved. Now, who wants to play Dominion?"
Simply, WFRP does try very hard to be about the story being told. Unlike D&D, it isn't about crawling through dungeons and slaying monsters, racking up XP. It is about forcing the players to think, to make choices, to gather clues, and to solve puzzles. It is about prejudice, class distinctions, and bigotry. It is about Good vs Evil, as well as all the shades of grey in between. It is about making hard choices when there is no real right solution. To that end, cheapening the story by allowing a single event to throw everything off kilter is a disservice to the players.
Now, that said, there are a number of ways to handle a situation. Perhaps the NPC killed wasn't actually the big boss. Was he a patsy? A front? A doppleganger? In that way, you could allow the death, and make the PCs just *think* they killed the boss. Or else, a GM can come up with other creative ways. As mentioned, perhaps loyal followers throw themselves in front of the boss. Or, you make the Boss into a truly formidable foe, by beefing up their HP or otherwise making the ability for the PCs to kill them into more of a story event and use a tracker, etc.
All my opinions, of course.