This and the Joel on Software piece both have their heads in the write place...which is putting a lot more time into estimating by actually diving in and planning your tasks with more detail.

It's the trade off of investing a lot more time in your estimating process to say that you had more accurate estimates. If you can afford to pull that time from somewhere in your organization, I'm sure it does produce more accurate estimates.

The value proposition boils down to how often you're going to do that? Are you going to spend 8 hours every sprint to dive into estimates to try to make them more accurate? If you're making a decision between multiple projects, the time investment to decide which path to travel down the road will make more sense.

Allocating the time every 2 weeks on the path you're currently travelling though? Is the value still there in that situation any more than re-evaluating the weight of a story when it's being worked?

This is where I steer clear of hours. Formula's like that also disregard things like turnover on teams, learning curve between different team members, on boarding time, experience differences, etc. The time trade off is rarely there.