Answer
If tracheids and vessel elements were alive at maturity, they would not be hollow pipes. This would greatly impede the transport of water, slowing or stopping it as the water would have to cross plasma membranes a number of times.
Work Step by Step
Think of what flow through the pipes in your house would be like if you inserted an inflated balloon into each pipe—this is what it would be like if the cells of the xylem were alive at maturity.
