One of our founders at Solid Quality, Itzik Ben-Gan is known to most in the SQL Community, particularly for his passion around the T-SQL language.
He's recently written an amazing blog post that describes why these functions are needed, along with a plea for helping promote the ideas to the Microsoft product group via the Connect site.
Anyone with an interest in T-SQL should read the post as, by itself, it provides great insights into the language and the possibilities that some subtle changes could bring.
You'll find it here: http://www.sqlmag.com/blogs/puzzled-by-t-sql/tabid/1023/entryid/13085/Window-Functions-OVER-Clause-Help-Make-a-Difference.aspx
Highly recommended reading!