I think it is an important question and anyone who trades should give it serious consideration. If the "edge" is a fallacy and prices are truly random, it is a bad bet to think you will succeed. Just like playing the slot machines there would be no possible strategy that could make the expected return positive, and though there may be winners in the short term, in the long run (very very long run) ALL traders would fail due to the cost of commissions. It's human nature to make order out of disorder, and people will try to find methods of beating it. But in the same way slot machine strategies are doomed to fail, so will trading strategies, in the LONG run, IF the markets are random. This includes any "money management" schemes one might propose. In addition, a few traders would seem to experience long term success, even many years of steady gains. But in the same way flipping 30 heads in a row WILL occur given enough flips, so some traders would experience seemingly uncanny results. This would only be due to luck.
So the big question is, are the markets random? I don't know. I tend to think, and maybe this is a fools bias, that they behave more like a poker game, and thus with skill one can read the "tells" and exploit them. Like poker, it is so close to randomness that it takes special attentiveness to find the tells, and the vast majority will not find them.
In an effort to convince myself that the markets display some predictable behavior, I look at breakout plays of small cap stocks during the late 90's. I believe that at this time, breakout trades would yield non-random results. I conclude that if there is one example of predictable behavior, there are most likely others, and you can't say the markets are random.
One other thing to consider is the spread. In theory (maybe not in practice) you have just as much chance of making the spread as anyone (if you bid to buy and offer to sell, of course), and if the spread is larger than your commission, you should have a positive expected value of return.
As far as the posters on the other thread, forgive the haters. This is a tough business and it's easy to take your aggressions out on some dude in a message board. It is not only a smart question, but it is the smartest question. The answer will determine whether you play the game or not. I have had, and still have, the same question as you. I started in the mid 90's, before internet trading. My return is currently 53x my original investment, but I can expect that if the markets are random, I will be giving this back if trade long enough.