The specification currently states that the attribute, if specified, must have a value that contains no "LF" (U+000A) or "CR" (U+000D) characters.

I would like to request that this restricition is reconsidered for Textarea input fields which, by their nature, are designed to span multiple lines and a placeholder hint to show a user an example of the data to enter should also be able to span multiple lines.

An example of this requirement is where a textarea is used to create a list of items e.g.

Please enter areas of work:
    Area A
    Area B
    Area C

The specification suggests the alternative use of Title but in most browsers this does not display in as helpful a manner and is, anyway, currently discouraged by the W3C, see:

http://dev.w3.org/html5/spec/global-attributes.html#attr-title
Relying on the title attribute is currently discouraged as many user agents do not expose the attribute in an accessible manner as required by this specification 

The use of this feature is becoming increasingly common, there is a clear demand for this feature to be supported (as can be seen by a quick Google search) and the decision to disallow line breaks imposes a decision that should really be left to individual website developers as to what best suits their own requirements.