(1) You surrender your content to Pinterest.
(2) Your visitors will see your content on Pinterest and no longer need your website.
(3) Your content can be hotlinked from Pinterest to anywhere on the web without your knowledge or consent.
(4) Your website will compete against its own content displayed on Pinterest in the SERPs.
(5) The pinterest.com/source/yourwebsite.com is an aggregate of your pinned content and tends to rank rather highly.
(6) Pinterest links are nofollow, it's shooting oneself in the foot before the race gets started.