Never happen. Thats like saying should theatres get rid of the different sizes for pop/popcorn and just have one size? They want you to buy the large, thats why its the best deal, they need the overpriced small there to convince you of that. However, if the large is too big/expensive then the small is always there so they don't loose your money.
But what you don't understand is that as it currently stands, there's no overlap whatsoever between the MacBook and MacBook Pro line.
They name is arbitrary.
You have:
A 13" laptop
A 15" laptop
A 17" laptop
Does it matter what they're called? They're basically one product line (Apple laptops) with different names (MacBook and MacBook Pro) for the sake of having different names.
Now, if there was a 15" MacBook Pro, some of you may be onto something... but there isn't