C'mon, ATC the world over is full of negotiation and letting aircraft do what they want when that option is available. They get to pick the level they want to fly on the route they want to fly at the speed they want to fly. It's called flight planning. "Request flight level....". "Request direct.....". "Request ILS approach......". Sounds a lot like negotiation to me.
You don't control their descent point unless it's required. You don't control their rate of climb unless it's required. You don't control their descent speed unless it's required. You don't control how they meet their fix time unless it's required. I'm not flying their aircraft. If they request 20 miles left due weather I don't vector unless required.
How is letting an aircraft decide how much time they want to lose with speed and the rest with vectors or holding any different? I'm still controlling things. If I can safely and sensibly accommodate a request then why shouldn't I? I guess I can wave my willie around and shout "thou shalt do it my way or the highway" but why bother when there's an option and I don't care which way it's done?