1.) The difference between 4 and 6 is noticeable in durability and ranged protection radius. It's the second thing that really makes this worth it IMO, though the first is rad too.
2.) I've heard that proficiency reduces your couch cooldown? May just be another rumor. In any case, with 5 WM @ level 33, you could make a pretty respectable hybrid between 1h and polearm (118/111 split or something more like 130/94), which would add a lot of versatility, and versatility is FUN, especially for a high level build.
3.) Scientology.
If you ever got to 35 you could max every skill you use and do like a 130/115 prof split or something more even like 122/124.