Not so much less is more, what we find when we break down work efforts is that the sum of the smaller pieces is generally bigger than what we would have sized for the project as a whole. Typically our enthusiasm and optimism lead us to a scope for an entire effort that doesn't really take into account all the stumbling blocks, details and gotchas. By breaking down the work, and considering all the possible roadblocks at a more granular level, we find that the sum of the parts exceeds the assumed effort of the work when viewed as a whole.
This is very helpful to more accurately set work time involved in an effort. Rather than add some ambiguous buffer to the overall scope, we can take each segment of work and analyze it for potential problems then plan time to resolve those problems and bake that into the granularized scope. It also provides more refined balance to time (overage for one segment can be regained by not needing the "problem time" in another) that should present a better adherence to the overall budget (of time and money).
