I, however, have a slightly odd take on this argument (if I were a planner I would probably describe my stance as being Feuerabendian). My position is that I completely accept the value which planners and other specialists can add to the creative output of an agency - and I believe that varied groups of people are a good thing. But I believe our current, sequential approach to using different talents is not the way to get the most from our mix of talents.
In a single sentence my view is: "Planning + creative = good; planning > creative = bad."
In short, I believe that the way our business now tends to make "being interesting" subordinate to "being logical" is the single greatest reason why a lot of advertising is awful (and explains why the number of people who "believe the ads are as good as the programmes" has been in constant decline for more than 20 years).
So, in a sentence, I think both disciplines are equally valuable. But I think creativity needs affirmative action more than planning does. In particular, the assumption that planning always gets to work on the problem first shows an inherent bias in our thinking which is not only uncreative, it's also downright unscientific.
As one creative remarked to a planner: "You and I both drink from the same well of inspiration. The difference is that you get to piss in it first."