Why Do People "Pre-empt" With Seven Of A Suit In Bridge?
Generally speaking, if you have only seven points as in the example hand, your opponents will have the preponderance of strength.  Pre-empts are designed to eat up the bidding room that your opponents could otherwise use to zone in on the right contract.  If, as it sometimes happens, your partner has the strength  instead, the pre-empt is still good, as it paints a very accurate picture of your hand that your partner will use to determine the right contract.

If you get doubled and go down three or four, think about what your opponents will have.  They will usually have missed a slam, and your result will compare favorably.  

It will happen, but it is very rare that your pre-empt will be a total disaster.