I am far from the authority on answering that question, but feel like I should chime in anyway.
Icebridge works with the Cryosat group, the are VERY interested in the data, hence the Croysat orbit grid that was flown Wednesday. Every instrument needs calibration and validation, including Cryosat itself, this is one area where the airborne campaign can help.
The funding was not in place for an immediate launch of another Icesat. Airborne measurements of some of these flight lines have been going on since the early 1990's and these repeat measurements can and do still tell a story. Sure an aircraft does not have the coverage of a satellite but that is one reason it is focused on specific areas.
The cost of an airborne campaign is also extremely less expensive than a satallite. The cost of the next Icesat could fund airborne campaigns to the Arctic/Antarctic for the next 50 years. With the status of NASA funding I feel they are getting quite the value when it comes to data point for the dollar.
The Icebridge/Icesat data is also freely released to the public @ NSIDC.
All the games don't build confidence: if it's not reproducible it's not science.
There are stringent calibrations done on every flight and pre and post campaign. These are also cross checked to Cryosat/old Icesat data for validation. Reproduction of calibration goals are <10cm (actually strive for <5cm) for surface elevations. There is instrument calibration documentation available as well if you really want to dive into the details. This group takes great pride in meeting these requirements, I assure you.
The P3 is back flying today and still nice weather here in Northern Greenland thanks to that nice HP Greenland block which is freezing most of the US!