If you are designing a business application with lots of entry forms and tabular data, writing code for your UI is both faster and more maintainable than using an UI designer. In those kinds of applications, there almost never need to precisely place one element at predefined place on screen, while on the other hand, there is a lots of repetition and design conventions that can simply be extracted into separate methods.
Take for example an Eclipse or OpenOffice preferences dialog. There are a bunch of categories and a bunch of different options to set for each. If you have to make something of that size, hand designing each screen is a mundane task. Much better approach is to write code which will generate UI elements on-the-fly, from provided domain data, according to some conventions and defaults.
I don't see how using a designer facilitates better separation, nor how that perceived separation would help with anything, given that you already keep your business logic out of the UI.
And, finally, if you're using Java, be sure to check out MigLayout. It works with Swing and SWT, its' syntax is very concise and clear, and it has a debug mode that's unbelievably useful.