Storing images as TIFF files is very space inefficient compared with raw, as TIFF images store three colours per pixel (at 8 or 16 bits per colour component, 24 or 48 total) compared to raw which just has the monochrome sensor data at 12 or 14 bits per pixel total. This monochrome data is interpolated into colour by exploiting the RGB colour filters placed in an alternating pattern over each pixel. To store the full range of colours available in the Raw you would need a 48bpp TIFF, which would take up about three times as much space (before compression).

Also raw preserves the maximum amount of editing flexibility - you're not commiting to any particular white balance or noise reduction setting. TIFFs are better than lossy JPEG images for archival purposes, but still not as good as raw.

I always keep the original Raw files, and keep a matching set of high quality JPEGs for easy viewing. There are arguments for using TIFF for archival purposes as it's an older, better documented format, understood by a much wider range of software. However if you're concerned about future compatibility then you can losslessly convert your proprietary Raw format images to Adobe Digital Negative files, which is an open format more likely to be supported in the future. The redundancy in an uncompressed 48bpp TIFF will make it slightly more tolerant of data errors, however. As Reid states there are better ways to guard against data loss, such as a backup system with error correcting codes, mirrored RAIDs etc.