It's quite simple, really. If you join today with some quite powerful system, say 1GH/s, it would take weeks to find a single block. However, the difficulty to find a single block is increasing in insane rates, so your chance to actually find your block will drop constantly, and most probably you will not find one in years.
If you mine in a pool you have enough assembled calculation power to find blocks here and now, and you'll get a smaller share - but you _will_ get a share!
So if you do not have half a data center of GPUs working for you, go for the pool.
i found this post to be the most convincing. along with that other one that u want to negate luck (ohyea luck/chaos is bad lol)