Actually WU is what you get paid for. U is denominated in whatever pool difficulty you have while WU is denominated in difficulty 1.
No, WU: includes rejects - which you do not get paid for.
U: is exactly what you get paid for - accepted shares.
I am mining at 1diff so in this case it is even the exact share count when multiplied by the elapsed time and divided by 60.
If you are not mining at 1diff then yes you don't know how much you will be paid, but it still does tell you how many payable shares you have submitted.
The API "Difficulty Accepted" will tell you exactly how many 1diff shares you have submitted.
Hm... OT, but I always thought U was adjusting to diff, so if you have a pool with varying difficulty, it wouldn't show accurate numbers, while WU, minus rejects, would always denominate in diff 1.
Oh well, one learns every day.
.b