That's a good question. I was asked this question during the last Parliament with regard to the practice of all governments of granting loans and then writing some of them off if the borrower defaults. This leaves parliamentarians out of the picture, since they never vote on those loans.
To give parliamentarians some control over debt levels and over these loans, which can result in write-offs and significant costs to the Crown, it would probably be advisable to allow parliamentarians to vote on the maximum limits to which debt can be increased by Crown corporations and government entities.
As you mentioned, the Crown does stand to make a net profit, albeit a small one, if the loans are granted at commercial rates. However, as we saw two years ago with Chrysler, sometimes the debt has to be written off. That means significant losses for the treasury that have not been formally approved by parliamentarians.
A mechanism allowing MPs and senators to vote on increases to loan limits would probably be appropriate. But that's obviously up to parliamentarians.