If, in the future, some sort of "better" licensing terms became available than the one we had agreed to, it would existing customers be given the option to switch to it (depending on the circumstances, of course?) With multiple products and licensing types and different products hovering around, I think there's probably situations where this may come up (the community suite, for instance).
While logically licensing costs seem to get more expensive with time, if some 'All IPB products for a low $5 with no renewal fees ever' deal came up might be nice to switch to ;)