I can understand in the 80's it was very common in TV to have a same actor play a different character in a series. But I can think of a few examples where the writers used the same actor play a SIMILAR character but chose to not really make them the same person, Ramon Franco for example, in blood brothers and Final Approach. He could have easily be written as the same person and Mac could recognize him. I wonder why they chose not to do this....

Because it was a different character.

Casting is done long after the episode is written in most cases, so unless it's a recurring role (which requires filming schedules to be worked out with the required actor) the writers usually have no idea who will end up playing the characters they write.

In this case it would have just been a standard episode which just happened to get Ram?n Franco reading for the role of Spider which ultimately he got.

Also, the 2 episodes were written by 2 different writers in 2 different countries 2 seasons appart. They don't keep records of every character used in every episode, so it's very unlikely Rick Drew (the writer of Blood Brothers) would have even known about the Ramon character in Final Approach.