I don't think you should worry about a correlation btwn writing and public speaking. One of my favorite non-fiction writers is Oliver Sacks, but he is one of the worst public speakers I have ever seen. I have seriously seen kids do book reports that were more coherent than he was. I do think having a good public persona can be good for PR though. I was more excited about Jasper Fforde's books after attending a reading and getting to meet him. I had to remind myself how much I enjoy Maeve Binchy's writing after getting close enough to smell her (EWWWWWW!).