This isn't about punishing DPS players. The whole basis for this is the assumption that at launch dps queues will climb substantially. Yoshi P is right that EVENTUALLY it will balance out but at launch its likely to go nuts. Most healers and tanks have a dps offspec simply because a lot of solo content is easier to do as a dps. Questing as a healer is boring and slow. Its very likely that a fair number of them will try out the new jobs of the back, particularly those who find the roles appealing thematically. That will likely cause a spike in the number of dps levelling in ratio to healers and tanks right of the bat. Those healers and tanks that don't and level straight away will probably go back and start levelling their dps off spec once they hit 70.
In time things will settle down and people will return to tanking and healing, if for no other reason than to get shorter queues. However at launch there will likely be a spike in the shortage of tanks and healers for dungeons. This is what this is all about. Tanks and Healers considering offering their time to dps in queues to fast track their queues. Tanks and Healers will have no trouble doing the MSQ dungeons and such right off the bat. If you want them doing it more than once they will probably mostly need a reason to.
No Yoshi P might be completely right and there will be no problems at all in which case you wont see hiring a tank happening, at least for levelling, and all the discussion in this thread will not matter. But if queues jump like a lot of people suspect then tanks and healers hiring themselves out to help dps get through queue times will actually help the situation. The alternative is the tanks and healers just queue for the stuff they want to do, usually only a few times and everyone else has to wait.
As for balance.... I will believe it when I see it. I think there is fundamental issues with how they seem to be thinking about balance. In the current meta, the only way to make PLD or DRK competitive with a WAR as an off tank will end up leading to Job Homogenisation with none of the tanks having any particular strengths. Frankly it might be easier if they outright make WAR an inferior MT and DRK and PLD inferior OTs and give WAR another tank Job to compete with. The primary values you want in a MT vs OT are quite different just as is the case with Main healers and Off healers.