The fact you can't join a pool is discouraging to those who think they may never win a block . Being in a pool provides a sense of security that you will get rewarded for your efforts as opposed to your own personal luck. Whether this is true or not is irrelevant, it's just the perception of it being true.
In terms of probability you are wrong when you take the time factor out, unless it's too difficult for you to find a block within a time frame you're comfortable with then I guess a pool would be the only option, otherwise right now even a crappy dual core can find a block within a week.
You are right about the probability of it, but the op is right about the human psychology of it. People quickly get discouraged when their dual-core celeron doesn't produce anything in many hours or even a few days. At some point in the long run -- that is, as the coin scales to perhaps thousands of miners -- the number of discouraged users giving up equals the number of new miners joining in. Miner population peaks at that point. Obviously, the higher the market price, the larger that equilibrium number will be. And, of course, there are other factors that affect where the balance point goes.
We may not be able to predict what that number will be, but we can observe (some of) the effects that factor into it.