Although I know that it's a bad idea to direct it, I can't help but thinking that it's sometimes necessary.  Especially in thrillers or horror movies, where the direction can be crucial to the scare/thrill.  And if the writer is not working with the director from the start (alot of horor movies have the same director as writer) the movie might not seem impressive to the reader.  
For instance, take a scene such as the school yard scene in Hitchcock's "the birds" (one of the best suspense scenes in cinema history, IMO).  The cutting between the two (and later three) images is what makes it suspensful.  If that were to be written regularly, here's how it would be:

Tippi Hedren (can't remember character name) sits on a bench.  Behind her, birds are gathering on the playground.  She smokes a cigarette as they continue to gather.  She sees a bird flying to the sky, and follows it with her eyes to the playground which is now covered with birds!

Without Hitchcock's direction of cutting between showing the birds gathering and showing a close-up of Tippi's face, half the suspense is lost.

So, are writers "allowed" to direct or semi-direct the movie when it is necessary to get the feel of the movie?

To put it simply, yes.

You write your description to "show" what is most important at the time. The director will direct to what the description says (for the most part). A director will be able to see what you were going for in the script and direct the movie accordingly. Only a poor director would miss the suspenseful buildup of the birds collecting outside the school.