I can't be sure why it was done but I think the balance patches are designed for endgame players/endgame gear. which leads me to the following conclusion:
Imagine having:
(Self buffed)
trans upg club with 50% attackspeed
trans stone 38% attackspeed
opaniel guarder 15% attackspeed
silva pet 30% attackspeed
In Party:
(party buffs from priest/rumir grant attackspeed too or the savage totem)
- Now imagine a templar on max gear etc with dd pen can do 30-40k regular hits self buffed (in party this can go up to 80-100k damage a hit) and it's able to spew out hits at like what 350-380 attackspeed or so self buffed and even faster in party? which means you attack at nearly 3 times a second which means you have a damage output of 3 times 35k is about a 100k damage a second. slightly slower attackspeed was probably introduced to lower this DPS (damage per second)
- Templar's AOE attacks became much stronger since the chance for them to AOE became 100% instead of 50%, which means surrounding targets, but also their effects such as euphoria and group euphoria will always trigger on AOE hits (this effects healing is exponentially increased with the amount of targets hit). slightly slower attackspeed was probably introduced to counter this
If you would ask me, templar should have a 0% boost in attackspeed on their blunt mastery since they get plenty of boosts and their weapon is already ''fast''. Not sure if you can follow my reasoning and not sure if this is the reason they decreased the attackspeed but it does make sense, at least it does so to me.