For me the point was to preserve the exact versions that are recorded on my fond tapes. Would probably have elected to do it myself even if there had been some way to reliably match whatever can be found out there with whats physically sitting on my shelf.
I'm surprised that your tapes still work as low-rate mp3. My guess would have been that the psycho-acoustics of a lossy compression scheme would lose a bunch of detail irrelevant to your ear (still sounds like screeching even if its muffled a bit) yet relevant to the data. If one wanted to keep c64 tapes around in .mp3 format, it could still be a good idea to convert any recordings to .tap, to optimize them with Finaltap and then to synthesize a clean waveform prior to compression. One will also probably want to have them around as .tap for emulator use.