737-600 should not be considered a dud even if orders seem rather low compared to -700 and -800.
737-600 is exactly a shortened 737-700, no more no less.
When second hand market of similar sized classic 737's runs dry in a few years time, then 737-600 orders will pick up.
Anyway the -600 will easily turn in profit at Seatle since "development" was little more than mating an existing classic 737 fuselage to 737NG wing and technology.
There is a need for airliners of all sizes. And there are thousands of planes of -600 size flying every day. When the old DC-9s and 737-200 finally has to be retired, then 737-600 will fill that gap.
The competing A318 will not fly until a few years into the future. I think that the timing of the A318 introduction was wisely closen. The "early" introduction of the 737-600 on the other hand may simply be because SAS wanted some 50 of them right away, and since the -700 already was there, then those -600's could be made almost overnight.
Always keep your number of landings equal to your number of take-offs