If you look at what a project looks like
here is a quick presentation of the Visual Studio 2010 editions per role
Who needs VS Professional?
VS Professional will come automaticcaly with the platform.,I mean if you have decided to use .NET 4.0 or Sharepoint or any component of the Microsoft platform, then the first tool you need is an editor.. at the end of the day, VS Pro is the editor to write any code on the Microsoft platform.
Now, do a developer "just" need VS Pro ? No. I don;t recommend that.
As an average, a developer will spend less then 20% of his time writing code, the rest will be a mix of discussion, debugging, installation, research ..... So do you have the tools to support your developers when writing quality code (not just writing code) and the 80% rest (running test, fixing bugs, reviewing code, managing DB, validating requirements ....)?
I usually present VS Pro as "basic" (only writing code), VS Premium as "Standard" (include the code quality tools and the DB management tools) and VS Ultimate as "advanced" (Architecure and testing tools, prototyping ...).
If you are wondering what is the rigth level of Visual Studio and you are a bit confused on the capabilities benefits for you, think about one thing ..... where is your cost (or where is your risk) ? In Australia (probably similar in North America, Europe,Japan ...) a developer cost around 1000$ per day (full position), so if he's working 220 days per year, a company is spending 220 000$ for the position.
How much cost VS 2010 ultimate per year ? let say 8 000$ .... that represent less then 4% of the cost.
The tools to support a developer is less then 4% of the cost .... so it make sense to have the best tooling set to support any component of the life a a developer and not just an IDE to write code.
What is designer role?. Is it similar to architect role?.