[development] Forms API #id questions

[Robert Douglass]

I'm in the habit of giving nearly every form element an #id because I've 
been doing lots of AJAX stuff and always have the need to grab things in 
the DOM. Can someone comment on this bit of instruction:
http://drupaldocs.org/api/head/file/contributions/docs/developer/topics/forms_api_reference.html#id

Is the use of #id discouraged?

Why is there no #id property allowed on hidden form elements?

Thanks
Robert