What's to not like, is that if they take it for granted that you'll do what you're told, then it might as well be controlled airspace. There's enough overt airspace grabbing without co-operating/enabling a covert airspace grab.
In practice I can't see myself refusing a reasonable request that's likely to improve my safety and that of someone else. But I think it's reasonable to expect it to be phrased as a request rather than an order.
Secondly, at least when overt airspace grabs are granted, you know where the pinch points are. If controllers take to manipulating class G traffic in an opaque and haphazard manner, it's 1) going to make flight planning more difficult and 2) could conceivably lead to unanticipated pinch points or other airspace issues that aren't subject to any scrutiny.