It depends, it is quite easy to get an r3 insc tactics/mot/command AL10 (max for r3 is 11) so if your going 12 12 and have 6 points left over it would be good if you could go part W or P for that type of shield. Sure its only 2 more armor, but it is more.
^ given that the base armor on the shield is 8 or more, ofcourse
Sure, there are a few exceptions. For a more exhaustive description of what happens to the armor rating of shields when you don't meet the requirement, see the wiki page on shields, specifically the second paragraph on that page.