A guide doesn't really do anything but stop you from thinking for yourself. If you know about all the items in the game, you should be able to figure out for yourself which item would be most ideal for the game that you are in. For example - if you are playing AD ranged... When do you get Black Cleaver instead of Last Whisper? When do you get Guardian's Angel instead of Banshees? When is Madred's viable? Should you get 2 doran's or one? Or none?
AD ranged is the MOST cookie cutter role in LoL and it still has some of these situational questions. If you are solo top or solo mid then you have a lot more to think about because your item build is going to vary a lot more based on who you are laning against and who their jungler is and what items they are getting. Your ward placements are going to vary depending on who the enemy jungler is or what your jungler is doing.
Reading a guide is about as valuable as choosing the recommended items every game.
While I agree with some of your thoughts, I don't agree with all of them. When I read a guide, I use it to get a feel for the champion knowing I am building him in a somewhat viable way. If I am new to the champ, I never use him in PVP until I am very comfortable with using him.
When I look at guides, I think about them very carefully. Why do they select certain runes or masteries? How do they benefit from it? If we assume that the champion in question is able to solo top, would runes or masteries change a lot? Or how about the start of the build?
For example, my favourite champ Wukong is able to solo top. When a champ solo tops, they either go for 1) A Dorans Blade / Shield, 2) Boots + 3 pots, 3) Cloth armour + 5 pots. If I go solo top as Wukong, I don't go for any of these. I go for Regrowth Pendant + a single potion. Why? Because I know that the strategy of beating a solo top is to make sure the lane does not get pushed to his tower where he can farm stupendously fast. You do this in such a way that you can harass the solo top and deny him farm and experience to level up. Against any of those 3 item selections that I mentioned above, this strategy is very effective. What if you get hurt too much as solo top? How are you gonna heal yourself from the damage that they did? What if you run out of potions or only have the Dorans Blade?
The fact is that I know it gets more complicated for you this way when you have these starting items. Whereas Regrowth Pendant + Pot allows me to stay alive and lane for a long period of time, even if I get harassed.
I suppose the point I am getting at is that I think you are assuming far too much when you say that 'a guide doesn't really do anything but stop you from thinking for yourself'. Only short-minded people really do this. I take the guides that I find the most useful and mix them up and just try them out to see if they work. Doesn't work? Change it some more until I find something that works for me.
Guides are useful for the fact that people can benefit from it. Just because some people don't utilize them properly doesn't mean that guides are useless or that they are for noobs.