The group is telling the story. I'm not going to let a good idea go to waste just because it isn't in the head of the player acting at the moment.
This is probably just different DMing styles.
The group is telling a story, but stories can go down rabbit holes.
When the DM allows most every good idea that comes along to exist, then stories can splinter off into many directions.
As a player or DM, I have enough to remember without remembering most of the hundreds of good ideas that came along in a years worth of gaming.
The game seems cleaner if there is one or two major plots and a small number of subplots. The problem with "Just saying yes" and allowing most good ideas that come along to work is that it muddies the waters. There's no problem with good ideas that are temporary, but the ones that will affect the game for quite a while should be taken with a grain of salt by the DM.
I know that most of the players at my game have day jobs and/or go to school. They have lives outside of gaming, so their shared imaginative experience tends to be fairly simple with straightforward goals. Broad ideas that are repeated over time so that everyone gets a good handle on them.
Take the example of the Wizard's Guild in the DMG. On the surface, it sounds ok. But problems can come in if the players continue to add elements to that small town. Suddenly, there is a Temple and a Fish Market and before you know it, there's a small city there instead of a small town. 6 real months later, the PCs get back to that small town and a player states that he is going to the Fish Market and the DM doesn't remember anything about the Fish Market or the NPCs that he created on the fly there. Now, this is a bit of an extreme example to illustrate the point, but the idea is to NOT take every good idea that comes along and incorporate it into the game.
Sometimes, make the players struggle a bit to accomplish their goals. Don't just hand out the option on a silver platter because a player thought of it (regardless of whether that option turns out to be successful or not).
Instead of "Just say Yes", "Just say Maybe".
Instead of being at one extreme of the wide open group shared experience or the opposite extreme of the extremely narrow group shared experience, be somewhere in the middle. As DM, allow some things. Disallow some other things.
4E has a strong push within the DMG to focus the game on the desires of the players almost to the point where the players are taking control (an almost player entitlement mentality). I personally believe in a more balanced approach. The players roleplay their PCs and come up with good ideas, and it's the DM's job to use some of those ideas and discard others to make the world seem plausible (not realistic, plausible). The world is not plausible if every time a player thinks that there should be a Wizard's Guild or a Magic Item Shop in a town they get to, there is one. The DM should sometimes allow such a thing and other times, nope. You have to come up with a different solution.
This challenges the players and makes them feel like the campaign world does not revolve solely around the PCs. Just saying Yes But every time makes it seem like a game solely for the benefits of the players and the experience is less satisfying that way.
Sometimes, you say to the player "your PC knows that she's probably not capable enough to climb down the rope with a shield in one hand and a weapon in the other, you can try, but your PC knows that she'll almost definitely fail". It's not a matter of assigning a penalty or making the task real difficult, it's a matter of shutting down a hairbrained idea before it starts. Some players will still be rebellious enough to try, but that's their choice. When the DM approaches this with "Yes, you can try that. But, it will be extremely difficult.", the player might not understand that extremely difficult means rolling a 20 on the dice for the Climb check twice in a row for the distance involved. Sometimes, it's better to just cut to the chase and let the player know that it's a really bad idea. Players should be able to figure this out on their own before even expressing the idea, but some of them don't.
And getting back to your point of not allowing a good idea because the player on stage at the moment didn't think of it, step back and think about that. The player who is on stage at the moment is effectively upstaged by someone else at the table. It's probably not a major thing, but for someone like myself who likes to give each player their fair share of table time and their fair share of solving problems or adding to the narrative, it's can sometimes be mildly annoying when some other player pipes in. Granted, it's a game with a group of people and a certain amount of that is unavoidable, but I do have strong opinions on letting each player be in the spotlight at the game.
Each player should get a chance to blossom at the table and I'll shut down any side idea no matter how good if it detracts with what that individual player is currently trying. Desires/ideas of the individual over the desires/ideas of the group. Over time, the group will meet consensus on a lot of things. But the fun of the moment is focused on the player in the spotlight. It's the ideas that that player expresses which I will be mostly be taking into consideration at that point in time.
(Ex. "Can Raven Crowking play in our game this week?")
