I agree with you there is a business strategy reason for pre-announcing, but it doesn't make your product better, it's just an attempt to freeze sales of competitors, and I don't think it works except in very special circumstances. For the market to care, you have to announce something that is going to be better/cheaper, and you have to have a track record in that space of delivering quality products.
Microsoft has no VR/AR track record, there isn't a VR/AR market to speak of, it has a track history of announcing then canceling products (every tablet ever until the Surface), of shipping disappointing first release products (every product ever including the Surface and Xbox), etc. Pre-announcing Hololens is giving away every innovative idea to competitors well ahead of launch, even ahead of fixing the problems with the product.
Microsoft has massive resources, it can silently develop hololens for years until it's truly ready, and when it launches it will make a big splash. No one needed to be teased about the iPod.