I doubt MrPippy is going to update it for new kernels etc, that's why he linked that in his instructions for you guys to update to the latest if you want. His work was done on the rootfs.img which had the RIL info we needed for CDMA Voice/Data. As changes progress making more adjustments to the CDMA specific stuff etc he will probably update the 1st post accordingly. Or any other major enhancements like Audio, GPS or Camera etc. I could see him updating then, even if it is in the Kernel or a new init. So I would just get in the habit of reading the changelogs at the Glemsom site and see if you want to update your kernel and modules or not.
And yes, the data.img is your actual filesystem, moreso like your home directory in Linux. So unless major enhancements warrant it, or you start experiencing issues, you ideally should be able to keep that and just update the zimage, modules, init and rootfs as needed. But if you are having trouble then of course you might have to forego your data.img and let it recreate it to fix any bugs. And some enhancements change the way things are written/updated so that's when you would have to delete that file as well.
Hope that helps enlighten things. Enjoy your Android, I know I am!