Well, Tom Baker's been doing all those Big Finish stories for years, so it makes sense he'll finish this one.
Although I'm not thrilled that resources will be wasted on a story that I've already enjoyed on animated form (love him or hate, Levine did make a proper version of Shada to see, and I enjoyed it immensly) and would rather they spent money on a Troughton or Hartnell story instead... nevertheless, its Tom Baker completing his half-finished story. So, for that alone, totally worth it.
Plus, I hope they really ditch that awful McCulough score. Horrible.