I guess that this US$ 300 price bracket that you refer to is the US$ 999 - US$ 1,299 range which includes the 11" MBA (both versions), the 13" MBP (lower-end version) and the 13" MBA (lower end version).
Apple has to offer different products at this price range because most people may only afford to buy computers at these kind of prices.
A recent report featured in Mac Rumors (
https://www.macrumors.com/2012/04/23/apple-predicted-to-discontinue-17-inch-macbook-pro/) put total sales of MacBooks at 3.1 million units.
Of the 3.1 million MacBooks Apple sold, 2.6 million were estimated to be 11" MBAs, 13" MBAs and 13" MBPs. Apple offers six different versions of these laptops, and four of them are at this price range. I bet that these four cheaper versions sell much more than the two higher-priced ones. More expensive machines are a tough sale, even for Apple: it hasn't even managed to sell 600,000 15" and 17" MBPs.
But, even if the 1.5 million units of the 13" MPB and the 1.1 million units of the MBA were evenly split between all the versions (which they certainly don't, as the cheaper ones are probably the best-sellers), we would have sales of almost 1.6 million laptops at the price range of US$ 999 - US$ 1,299, which accounts to more than half of the total sales of MacBooks.
So Apple really needs laptops at this price range. Apple is a premium brand, and it needs its flagship models to show how mighty it is. It's marketing. But it also needs the cheaper models because everybody wants to be at Apple's party and just a few are able to afford the top-of-line Macs. The cheaper models drive sales. This is not a strategy of Apple: BMW and Mercedes-Benz, for instance, are known for their large luxury cars, but they sell lots of smaller-sized cars at more affordable prices because everybody wants a BMW or a Mercedes-Benz car as much as everybody wants an Apple product. It's how it works.
Said that, I think that the Sony Vaio Z is a great benchmark for Apple. The Vaio Z has a huge achievement for Sony, IMHO. It packs a powerful i5 or i7 standard-voltage processor with a 13" high resolution screen (1600x900 or 1920x1080), an SSD drive and a good battery in a package which is thinner and lighter than a MBA. It has everything a mobile computer should have: it is thin, light and portable, yet fast and powerful. It lacks an optical disk drive and a dedicated video card - both of which are in the separate Power Media Dock -, but that's a requirement for being thin and light. Still, I've read that the Vaio Z has some serious heating problems (but I know nobody that actually possesses one so I could check for myself).
But the Sony Vaio Z is a different beast. Sony has standard-sized 13" inch notebooks and it has also launched a 13" ultrabook. Both are priced lower than the Vaio Z, which a premium, top-of-the line, ultraportable not aimed at major audiences. Would Apple dare to go against the Vaio Z with the MBP? If it does, it will have to launch a higher-priced laptop (which would sell less) or to decrease its margins. Wouldn't it be too risky and bad for business?
One last note: we've taken 100 pages of this thread discussing a possible MBP redesign. People were eagerly waiting for a brand-new MPB in April, and it didn't come. Now people are waiting for the release of these laptops in June. Will they come? Instead of becoming more frequent, the rumors have just stopped. Perhaps we're just wasting our time here. Isn't it time to move on?
