Saving to “consumer” image formats allows Slicer to interact with a larger community of software and encourages people to use Slicer’s output for further analysis and software development. As far as a note why it should be fixed, I would think this is self-evident.
You lose a lot of information if you use consumer file formats (slice spacing, image position, orientation, other metadata) and you may have image artifacts due to compression, so images saved in these formats are not well suited for further analysis. ScreenCapture module can be used for exporting view contents to image files/videos for presentation. It is clear that it is a nice to be able to export to tiff, but for me it is not self-evident if this is really important.
What would you use the exported files for? How do you manage the lost metadata (image position, orientation, slice spacing, …)? What software you use to further process tiff files? Do these software support standard medical image computing file formats (nrrd, metaimage, nifti) or DICOM?