If you're not in debt your not paying interest to the bank and therefore hopefully spending it on actual stuff (goods and services) which all helps the country grow.
Although like most things it's closing the stable door after the horse has bolted, you should be trying to pay off debt at the end of the boom years having taken it out in the recession and early boom to limit the downturn and fund growth. Hence Brown's assertion athat over the economic cycle government borowing should be zero. His downfall was not calling the top of the market or if he did, not hikeing taxes up during the second half of labours tennure which would have been the right thing. The problem with doing the right thing is it usualy makes you unelectable.