Mercedes-Benz is notorious for running the "Pull-Ahead Program". BMW has the loyalty credit for a good 6 months out of the year, and Audi usually runs the "conquest" where they actually make your last payment on a competitor vehicle.
Other major brands, such as Toyota, VW, and Chrysler will offer lease cash and bonuses, IN ADDITION to loyalty credits.
Realistically speaking though, besides Mercedes-Benz and Porsche,
no one is actually just wiping your previous payment clean.
What would've been a super aggressive payment utilizing all credits and rolling no payments, usually becomes just aggressive, and distracts you from the money being folded in from your previous car.
That money is going to make it into your payment one way or another.
Most manufacturers even give limitations to how these credits are utilized. How much time is left on your lease is a major factor. Most brands won't offer such incentives if your lease isn't within 3-6 months of maturity.
Why do they do this? Most people start shopping for a new car 3 months before their lease end. With an offer that aggressive, most usually stay with the brand.
I will say one thing though: more often than not, you will get a sales manager that is more willing to take a further loss if he knows that your remaining payments will make the new number unattractive.
It still boils down to the same thing though, that money he is discounting off the new car could've happened with or without your old payments being involved.
Is this a bad thing? No..
Maybe if you didn't get out early then you'd have a ton of excess mileage charges?
Or you're utilizing a rebate or incentive during the holidays that normally wouldn't have been there, to help offset the payments you're rolling in?
However you choose to make it happen, the fact still remains:
More often than not, you are still paying your remaining payments in your new lease when you ask to get out early.