I think that leases should be used at the beginning to maximise turn advantage.  For example, if you can only afford 1000 credits, you can spend 1000 credits to rushbuy an improvement or spend 700 credits to rushbuy the improvement and spend the remaining 300 credits on rushbuying a ship.  You get one extra ship at the expense of lease payments.  If the ship is a colony ship, you get 1 extra planet earlier.

In DA I would rush buy the Mind Control Center, if for no other reason than you made more profit for the +100% econ bonus than you paid in interest for the loan.  Many times I wouldnt even care if I went below the -500 BC limit, as long as my per turn income would get me out of the hole in a few turns or so.