Concept sketching is important. Just like it's risky for a developer to start writing code without a plan, it's risky for a designer to start wire framing or visual design with out thinking it through first.
Sketching is also important for other reasons: Designers can quickly iterate and re-iterate over many ideas very quickly; clients and stakeholders can get very early input to basic layout and interaction ideas; the whole team can be engaged and consensus maintained throughout; and, not least, we can figure out where and when to present all those features to the user and see if we're trying to cram too much on the screen.