It says 'for hobbyist and students' not because there is some limitations, but rather because Express editions lacks certain features that may make commercial development more efficient and help to release higher quality product. For example, serious team development requires source code control system. Visual Studio does support integration with SCC while Express editions do not. You may also want to install add-ins like Resharper to make your code development more productive. Express editions do not support add-ins. Although it is completely possible to develop high quality Web site using only Notepad, extra tools make developers more efficient and hence Visual Studio is more appropriate since it effectively saves money in the long run.
Again - if you are willing to use Express edition and live with its limitation, you can. There are no legal restrictions.
Thanks
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