Agility unfortunately doesn't add as much as strength does since strength gives HP as well as dmg. Its why so many people are Strength builds.
Agility makes you move a little faster as Tom said, and it unlocks the ability to have more WM as you probably know.
WM is debated as being mostly unnecessary, but it depends on how you like things. WM does make you swing a BIT faster, and it adds 1.5 DMG per every 10 WPF you get from it. So not nearly as much as straight up PS but it comes with the extra bit of speed boost on your swings, parry, etc.
I tend to go 18/21 with MW Longsword, I like it....6 PS, 7 ATH, 7 WM, couple IF
This gen though im going to lvl 32 and will be 21/21 for the 7th PS
- ULTIMATELY - you gotta test it out in game. Some people say go super strength no WM, others say no you need ATH over 6 and you need some WM, etc....
Gotta test it out in game and see what you are comfortable with - everyone's fighting style is different.