Waiting on perfect can be costly. If you take forever trying to get a deal done to work on the last parts of minutiae, the deal can walk. If you trade time for a perfect presentation of a website, the enthusiasm from the customer can be much lower than the initial hype and hope.
Perfect is just a snapshot. So, if you put all your eggs in one basket looking for the masterpiece, it can be ruined when real life tests the robustness of your idea. You may have to start all over where the effort could have been saved if you chose a path of iteration and learning.
Of course, there are areas where the trade-off to take your time and make things perfect is worth the cost. Brain surgery, manned flights and trapeze artistry fall under this. For the work that can morph and endure the rigor of agile testing, why wait for perfect? Pursue perfection with speed. Get something out there and test it quickly. Empirical testing has a way of calibrating a design or an idea into what it should be.