The XNA sprite font processor uses DXT3 compression that does not work well with the new premultiplied alpha in XNA 4.0. Hence the blocky look of the text.
The Nuclex sprite font processor outputs exactly the same data, except that the internal
Texture2D that holds the font bitmap data is not compressed - so it has no quality issues.
Because the data from the Nuclex processor is in the same format, the XNA
SpriteFont reader can read it, and you do not need to include any Nuclex libraries for distribution with your game.
(It also conveniently allows you to use the same XML format as XNA's native sprite font definitions.)
That being said: there's nothing wrong with bundling external libraries with your game. It's not like they get installed on your system or anything. (You could even go so far as to merge the two assemblies).
And the same thing goes for the code (if you're worried about version control, for example). You can just put
Nuclex.Fonts.Content.TrueTypeImporter.dll in with your source code. It's actually not a big deal.