@kormoran – other’s may, and indeed do, disagree, but it depends on what your investing strategy is, and when you may need your money.
If you’re in your 20s and it’s not money that you’re going to need for a house deposit or wedding, then stick it all in a shares index tracker because if it crashes you’ve got plenty of time for it to recover and you’ll benefit from the increased returns that go with the higher risk.
However for other people, more diversification is likely to be beneficial. What you gain from the higher returns can be eat into by volatility, and so it makes sense to have a balance of different assets in your portfolio. So you split your investment between various trackers of stocks and bonds. However over time the different trackers grow at different rates, and so if you don’t keep an eye on them and rebalance them occasionally you’ll end up over-exposed to one particular sector.