It's not like that would be super broken. Spanish and Persians are both about the same strength throughout the rest of the game (Spanish better in castle age coz of conqs), so if Spanish having full trash isn't OP, then why would it be OP with the Persians?
Besides, having FU Elite Skirmishers would help a lot at using skirms to back up your cavalry in 1v1
It's about uniqueness which I am sure 97.1% of aoezone will agree with the more traditional civs had unique traits. Think about it
Teutons have no light cav
Turks no pike eSkirm Onager
Persia no 2h swordsmen
Saracen no cavalier
Malian with Arbs no bracer SO no SE
Spanish full trash, full dock and no Crossbowmen
Persian full stable
Korea full university
Incas full barracks
Saracen full archery range in all versions
Ethiopian full siege
You give Persians bracer. Suddenly Spanish lose their unique charm of FU trash even if giving Persia bracer is partially justified they already have their own traits.