It's only fair and practical to insist on 12 consecutive monthly payments.
If you were able to drop out for a month here and there, and then get back on the payment ladder at the same point, it would encourage those of us who don't use the program frequently or commercially to deliberately organise our projects and workflow to just pay for a month here and there when we required the latest features in the software.
Say for example, that providing you have a permanent licence for a reasonably recent version of SONAR, in the current model there's nothing to stop you from doing all your tracking and basic MIDI work on that older version. If you organise your workflow carefully, what's to stop you signing up with a single monthly payment to the very latest SONAR Platinum to mix all those projects, being able as you would, to take full advantage of all the cool features. After a month, you stop your payments and revert back to 'preparing' the next batch of products in X3, for example. In 4 or 5 months time, you simply pay another month and so on.
Under these circumstances, which as far as I can tell is completely legitimate, I think it would be completely unfair for someone following this method to keep the latest version of the software after they've maybe paid 12 separate monthly payments over what might be 3 or 4 years, or longer.