You must make sure that the inner text of a link (<a> tag) does not echo the original href of the link, because Marketo will rewrite the href and make it look like you're trying to fool the recipient.
Let me give an example. In an orignal email, this link:
<a href="http://example.com/mypage.html">http://example.com/mypage.html</a>
will be rewritten to the bounce off the tracking server:
<a href="http://go.example.com/randomcode123456">http://example.com/mypage.html</a>
and that looks malicious.
While using friendly text:
<a href="http://example.com/mypage.html">Click here</a>
won't give the same impression to the scanner, because there is no URL-like text.