> IsNullOrWhiteSpace() was added in dot net 4:

Look at what the helper actually does. I already addressed this in my comment.

     string hello = magic();    
     if(hello.IsNullOrWhiteSpace()) 
     { ... }

Is significantly more intuitive than:

     string hello = magic(); 
     if(string.IsNullOrWhiteSpace(hello))
     { ... }

Plus it is consistent with things like Contains("XYZ") (e.g. hello.Contains("something")).

Truncate is useful just because Substring() throws a bunch of dumb exceptions. It is mostly a workaround for Substring's poor design.