I’m from Egypt, and the coaches here always prefer buying the blades and rubber separately to buying a bar. Is there any rationale behind that?
The reason for buying the blade and rubbers separately is that you can be very specific about choosing a table tennis bat to match your playing style. All rubbers and blades have different playing characteristics. A player may want a very fast, spinny rubber for his forehand and a slower rubber on his backhand. Some players prefer fast blades, others prefer slow.
Often players at the beginner and intermediate level will choose a pre-made bat, meaning the rubbers are the same on both sides. This is a good choice as players are developing. But at some stage, players will have a better idea about their specific playing style and stroke mechanics and will then choose rubbers which best match how they play.