I am currently doing a course and it seems to focus a lot on JavaScript, is it a language I should really know if I want to be a good web designer? It seems kinda cool but I just don't see me really using it, a lot of what it seems to do can be covered by PHP which I pln on learning.
JS is more for behaviour and interaction, see Google apps such as Maps or GMail which partly or fully use JS for many functions. It's not something that needs that much of a focus in your course though, as you are suggesting.
Javascript is useful when it comes to usability. But as it's client side, should never be used as a primary security function. I'm a PHP coder and I always say 'It can be done with PHP' which is true, but users hate leaving the page when they press submit in this web 2.0 world. I recommend you look into AJAX