That's overstating it. It's okay to let things be "wasted" and often better than trying to shoehorn in things that you worked out. Almost without fail, if I do any prep for a session the bulk of it goes unused, basically because the end up not getting to what I planned for, going a different direction, or asking to do something I hadn't considered and must completely improvise.
I'm okay if some stuff I prepped gets "wasted." Most of my stuff does not, however, because I tailor my sessions to what the players told me they wanted to do at the end of the previous session, the only possible exception being the very first adventure. That being said, a DM who preps as much as I do can feel pressured to shoehorn it in. It's not something I do, but I've seen DMs do it, so I know it exists. It's something to watch out for.
Without much practice, monsters and skill challenges can be produced almost entirely on the fly.
In my opinion, a prepped 4e encounter with all the bells and whistles will generally have more challenge and impact than an improvised one. I don't hold the view that 4e is good for improvised games. To me, that's like towing a boat behind a Lamborghini - you might be able to do it, but it's not built for that. In 3.X and 4e, I see the set-piece as king.
It only becomes a railroad when they instead fixate on the weird mayor, or the woods in the opposite direction and the DM does everything in his or her power to get the PCs to the mansion.
I would have started them in the mansion, in the middle of compelling action. If they want to explore the woods next session, no problem, I'll have an awesome adventure ready to run. Next session.
This is the worrisome mindset, the idea that if the PCs buck the adventure that's laid out for them, even if it was their idea, that they're "jerks." That certainly can be the case, but I don't think it's healthy to go in with the assumption that because you're all friends and all want to have fun, that everyone's just going to "go along" with what the DM planned.
I know if my friends did it, they were doing it to be jerks, because they know I write and prepare specifically for what they want to do. It's part of the social contract of the table - you can have free rein with whatever you want to do, but if you want it to be awesome, tell me at the end of the session what you want to do next session, and I'll prepare that for you, specifically. Veering off that path a bit, coming at the encounter from a different direction, or talking your way through it are all good "solutions" to the scene, but choosing to do something else entirely knowing I've spent a week prepping your game is, frankly, a jerk move.
Although Centauri and iserith seem to be speaking on opposite sides of the argument, I find myself agreeing with both of them. This is easy because both phrase/frame the set-up in such a way that makes it easy to knock it down. (They both use straw man arguments effectively, and I don't consider that to be a bad thing.) Their respective cautionary tales are worth heeding, and I would welcome either of them as my DM and both of them as my players.
Thanks for the feedback. How could I make my argument more effective? (I ask that earnestly.)