BizTalk uses bits of these applications itself when certain features are installed. (To my mind, this lack of "re-inventing the wheel" is good, albeit it does feel a little strange to load Excel and Visual Studio on a server computer.)
The Visual Studio components are used to provide user interface functionality for writing code. According to at least one document I've read, if you're not going to develop code on the server computer (in Microsoft parlance a "production-only system") you can install w/o Visual Studio.
Excel is used in the "Business Activity Monitoring" client (BAM client). The libraries for the BAM functionality have always been "tightly coupled" to Excel. I expect that this is because they wanted to give users Excel's charting and pivot table functionality w/o having to code something Excel-like. If you're not going to use BAM on the server computer, I believe you can get away w/o having Excel installed (though I think you'll have to deselect the feature during install).
There's a handy (but poorly rendered) "Feature Dependency Matrix" here: http://www.microsoft.com/biztalk/en/us/system-requirements.aspx