The web designer is wrong in thinking that because you have separate
edit and new views files that most of the page is different and not
sharing a greate deal of code. They are ignoring the fact that most
template view are views within views. Especially if you are using a
partial - where you have a template within a template within a template.
The outer template is the layout. Into this is inserted the view. And
into this is inserted the form partial. In effect you have:
layout | view | partial | view | layout
Where the partial is within the view which is within the layout. A
classic onion skin type arrangement. With your two views what you
effectively have are:
layout | edit | form | edit | layout
layout | new | form | new | layout
So by choosing a different view you are doing one big if. That is if
view is 'edit' slot the 'edit' template in the middle layer, or if the
view is 'new' slot the 'new' template into the middle layer.
What's more this is far neater than using lots of if statements with a
single template. You've separated the parts that need to be different
into two separate files, so far less chance that you'll confuse the two
actions : much less likely that something that should only occur in a
new action will pop up in an edit action.
You've also reduced the number of times you need to run the if. Just
once - to decide which middle template to use, rather than at every
point in the single template where a choice needs to be made.
I think your developer is demonstrating a fundamental lack of
understanding about how Rails templates work.