If iMessages have benefits (they do) then there is a technical reason to show you the bubble colors - so you know the benefits apply. If sending video to a blue contact is better than sending it to green, there's a reason to know.

Does it ACTUALLY matter? Maybe not? But people really do complain about a non-iPhone "degrading" a group chat, so it is indicating something.

At the time they made iMessage at first? It was likely a real advance and only because they could control both ends. But now? They may be large enough that it's unfair use of their monopoly in one area to affect another, and get slapped or forced to interoperate.