Mesaba is hiring and training to staff the airline. If there wasn't a need, they wouldn't be doing it. Training costs aren't exactly cheap. At the moment, the saab program is the only one that is run in house. The 200 and 900 are contracted out to Bombardier in YUL and CAE in Denver. These contracts will likely end right around the same time the major hiring stops. At that point the 900 sim will join the 200 sim in MSP and our own instructors will be doing the training.
During our last down turn after 9/11, Ch11 and the subsequent lack of hiring for so many years, most the instructors went back to fly the line. That's why they have phantom positions. There will always be activity in the training department for recurrent ground schools, PCs and RFTs.
With only 17 200s, the numbers are pretty much there for staffing, much like the saab. The 900 is the biggest fleet and requires the most pilots, therefore that fleet will still get staffed. I wouldn't expect huge classes for much longer though, we've got to be nearing the end. Classes are projected into the future and their sizes depend on the staffing needs of the company. There are many classes that are already filled through the summer, so these vacancies that are coming out now are for newhires that have interviewed already, and probably have been sitting for a bit waiting for class.
As more people leave the saab to go to the jets, the saab numbers will come down, since they're not backfilling. At some point they'll have to start putting people back in the saab to maintain the numbers for that fleet.