It's weird when you look at the points distribution. After 7, there is only one perk at level 8 that unlocks (the last level of parkour). Level 9 doesn't unlock perks and only gives you a boost in headshots damage and dismember chance.
Agility 10 though unlocks the last level of the 3 combat weapon perks and the two stealth perks.
I play SP and I tend to use Agility more than anything else, but I spread out my points so I am always investing into the other attribute trees to unlock supporting perks. I do occasionally do a pure attribute playthrough or sometimes limit myself to only the first unlock perks in other attribute trees at level 1.