As I understand, you are talking about Special Ability (SA)
1. To be able to learn one SA, player need 40 or 50 Skill Point (SP)
2. One quality = 15 SPs there fore 3Qs = 45 SPs
So, we can say two players, same quality but the one with SA could be consider 3Q higher than the other. Your Choice.
Like you said.. When a player most benefits with SA eg: AMC/ST with Free Kick or 1v1 SA that would be awesome because this player probably has very high Shooting stat which most benefit with his Free Kick or 1v1 SA!
On the other hand, you don't want your DC to have Conner Taker SA as his Crossing Stat would not be so good!
Playmaker is pretty much useless, as I have seen so far.
Personally I think the most useful SAs are:Conner Taker, Free Kick, Aerial Defender and Defensive Wall.. then maybe 1v1, Shadow Striker, Dribbler.