There are a few similarities as to why a slate item may have been slavaged compared to why a flint tool would have been re-sharpened. Slate, like flint takes a great bit of work to produce a finished product. Like flint, It can be reworked into a new form that takes on a new function. Example: The bird effigy below was salvaged from a bannerstone and then engraved. Note the eye created by the banding.
That is where the similarities end. Unlike flint, slate was used for utility (bannerstones) as well as ornamentation or ceremonial items (gorgets, pendants, birdstones, and so on). The use of tally marks, drilling, using the banding of the slate to enhance its beauty, and engraving were all used in the process of creating a slate item. Flint was generally re-sharpened in order to extend it's life as a utility tool, while salvaging slate was most likely done for other reasons. A few possible reasons include: Did engraving the slate give it powers? Could it ward off evil spirits? Was it an offering to the good spirits? Was it a status item within the group? or as it just a beautiful object? These questions will most likely never be fully answered.