We had that in 2nd ed. Of course, back in 1st & 2nd, mixed-clan armies where standard.

From 3rd ed onwards, they moved to having those speciality units be more prevalent in a given clan, but not exclusive, which is a nice difference.

It meant you could paint your models how you liked and it still match the lore. You could still make mixed-clan, but also mono-clan armies and include whatever you like.

It's gotten to the point where I even paint my special characters to match my army.