I disagree **strongly** with limiting the ability to paste Google links into
 comments in any way. 
There is a category of brain-dead, exceedingly trivial questions, that can be answered by a link to the proper, authoritative page in the respective manual ("How do I replace a string in PHP?"). In those cases - and only those! - informing the OP of the existence of search engines is perfectly okay in my eyes.

Sometimes, it is right to comment "Just Google it" and a (polite) RTFM in the form of a manual link. Not always, mind, and it's always good to err on the lenient side. But sometimes, it really is.

*Answers* consisting of just a Google link or a mere RTFM are not okay, for the reasons you state. But as @dmckee points out,  those are usually downvoted into the ground anyway. 

[Current example][1] where the user should have searched SO


  [1]: http://stackoverflow.com/questions/5696263/php-convert-date-to-certain-format