What Matey says, I felt a big difference with 7 vs 8 athl, plus with 4 ps, I expected to do very little damage, and compensated with movement, but with 15/21 I felt like I lacked damage. I know it is a mental issue, since I did more damage most of the time, but that little damage was not worth it compared to the extra mobility for me.
Also, bucklers improve so much with shield skill, it lasted so much longer and covered arrows better with 8 shield, so with 7 I had to use elite cav and similar which slow you down even more
And my 12/24 was a backstabber, whereas with the 15/21 I had to stay with the main group much more often.