"Keep in mind that it gets harder the bigger you are"

Generally, this is true, but no one is doubting MSFT's profitability right now, the doubts are with regards to MSFT's future profitability.

"The flaw with speculating about the future is that anything that's certain will be priced into the stock immediately."

Only in the very short term and with short-term information that is both publicly available to all investors and verifiably true - most information does not meet these criteria. The longer you move out on your time horizon the more uncertainty will impact the market price.

There is an enormous amount of literature out there with relation to determining the future price movements of volatile assets in markets with asymmetrical information (not everyone gets all news at the same time). You should google the "efficient-market hypothesis" as well as recent efforts to test the assumptions that it relies on. (Thanks to the fall-out that occurred immediately after the 2008 market bust there has been a lot of soul-searching in the investment management industry.)

"A lot of people think they're being prudent just because they're investing with a \"long-term perspective\". That doesn't get you anything in and of itself - I can buy a car and hold onto it forever but it'll never appreciate (well, until it becomes an antique)."

They don't have to think that they are being prudent - there is evidence to demonstrate the prudence of this strategy. There is a large body of research out there that has demonstrated that long-term investment via index funds with low management fee overhead is a solid, profitable investment strategy.

"But that just makes it more important that you understand why you've chosen to hold onto this stock for so long. And I don't think speculation about the future is a good basis for that decision."

MSFT has underperformed over the past 5 and 10 year horizons. Past performance is not a guarantee of future results, but I have found it to be a worthwhile indicator to pay attention to.

I was a portfolio manager for about 3 years (before I decided to jump ship and try a startup). When building a portfolio for someone, one of the most important questions concerns the time horizon of the investment. A portfolio's risk profile should be appropriate for the investor. The only way to determine this is to look at the past performance, past volatility, and future prospects of any potential portfolio asset.

In the long-term everything is uncertain - I agree with you on this. However, where we disagree is that you seem to believe that all future events have an equivalent uncertainty, but this is demonstrably false.

There is a implicit probability associated with all future events, and these probabilities, in the aggregate, will impact today's share price. Some events are more probable than others. For a subtle demonstration of this, you could easily look at the performance of some of the larger pharmaceutical companies. As they get closer and closer to the expiration of some of their key money-making patents the volatility of the companies notably increases.

There is a considerable amount of truth to what you are saying, but I would, personally, hesitate to invest in an individual company without incorporating some type of business risk analysis into my decision.

Edit: Minor grammatical flaws. I apologize for any that remain, this was typed in a hurry.