The name Letha itself actually is actually related to oblivion, from the name of the river through hades Lethes. I double checked this on Wikipedia (under Latis, Letha is some comic book character in a metal bra) but I first heard it from someone named Letha.
While I may not agree with the first part of your comment, I agree with the second: this name will attract unwanted L's like the strongest magnet that ever existed.
I personally knew an older lady named Leatha (pronounced Lee-tha), and have seen it spelled Leitha as well. The usage of the name would have occurred 80-100 years ago.