If we had to guess, what is the rough percentage of effectiveness, in the real world, granted by having the best tools over the worst (but still useful) ones?
Obviously, it varies some with the tool. A simple claw hammer that won't break in your hand can't be much improved for a short piece of work. Improvements are nearly all aimed at making it fit in your hand (fatigue over long use) and durability. Something much more complex, such as a computer, has a wider range--though still a definite cut off absent skill.
As a tool example, this came up in conversation the other day. In non-software development companies (ie, they don't write software as a product), IT shops will restrict and hand-out very baseline computers to every user. Developers need better computers, because the tools to write software often need more resources than the software itself. As a result, there's a often a conflict between IT and in-house development staff who need better than IT-expected computers.
1GB ram on an XP PC may be fine for office staff. Totally won't cut it running Visual Studio 2010 (which is literally consuming 1GB of ram by itself on my PC as I write this).
Now using this metaphor of My PC = My D&D Sword of bonuses I want. One could argue that if all I have is a crappy PC, I should use a lighter developer tool. Technically I can write a program on the computer. However, to solve the problem at hand, I actually do need a more powerful PC.
The same goes for magic swords. Sure, I could stick to killing rats and goblins with my non-magic sword. But if I am actually confronted by a level appropriate monster that has magic requirements to hit (silver, +1, etc), then I actually do need a proper magic weapon.
D&D has ALWAYS suffered from this concept. Werewolves always needed magic or silver to-hit. Other monsters always needed magic of a minimum + to hit. This meant that if the PCs encountered it, they HAD to have the item or were probably screwed.
Which leads us to, which came first, the weapon, the monster, news of the monster, news of the weapon?
If the PCs go looking for the monster without the right weapon, that's their fault, but then odds are good experienced players aren't going to do that.
If they go looking for the weapon they NEED so they can kill the monster, this is effectively WISHLIST behavior. They are telling the GM they want a certain weapon by seeking it out in the game.
If the PCs just run into the monster, it's the GMs goal to kill/thwart them, since he knows they don't have the weapon to deal with it.
If the PCs get news of the monster, they have the opportunity to decide whether to avoid it, or go seek out the proper weapon to defeat it.
Part of the christmas tree effect, is monsters that NEED specific weapons to kill them. It reinforces the requirement that PCs of a certain level be carrying +X weapons because all the monsters at that level require it.