And if recursion is usually slower what is the technical reason for ever using it over for loop iteration?
Because in some algorithms are hard to solve it iteratively. Try to solve depth-first search in both recursively and iteratively. You will get the idea that it is plain hard to solve DFS with iteration.
Another good thing to try out : Try to write Merge sort iteratively. It will take you quite some time.
Is it correct to say that everywhere recursion is used a for loop could be used?
Yes. This thread has a very good answer for this.
And if it is always possible to convert an recursion into a for loop is there a rule of thumb way to do it?
Trust me. Try to write your own version to solve depth-first search iteratively. You will notice that some problems are easier to solve it recursively.
Hint : Recursion is good when you are solving a problem that can be solved by divide and conquer technique.