I have another consideration for why nav controls should be available. I want to make web mapping a suitable alternative to the desktop client for my users who are not good candidates for using desktop clients. They still need to make maps to meet their needs, and to me that means having control over coordinate system and scale. And that means State Plane rather than Web Mercator as well as arbitrary scale rather than pre-defined tile levels. (Yes, I know this also means dynamic maps which take more time and resources). So, in order to offer control over zooming to an arbitrary scale, I think it's natural to want to select a zoom tool to drag an extent and then make fine adjustments as needed. That also implies the need for a pan tool since you need to have a way to switch functions. And I think, under these circumstances, that previous and next extent functions also help in this scenario.
Thanks,
Marc