If the Comsumer Price Index is imperfect then why do we use it?

Answer 1

Because nothing in this world is perfect!

Seriously, the biggest "flaw" in the consumer price index is that it doesn't factor in food and energy, which make up a large part of consumer spending. The theory is that they are volatile, changing with the season and with random events like hurricanes and wars.

On the other hand, if the price of energy or food changes and stays changed for a long time it will find its way into the index indirectly, as the cost of goods will rise.

Answer 2

Because it's easy enough for the nat stats bureau to calculate, and sufficiently good as a measure of inflation to base both policy and propaganda on.