Don't think your email can be altered in transit? Don't see a need for TLS? Find out how the Dalai Lama and US government computers have recently been hacked? The Dalai Lama had email attachments appearing to be from coworkers replaced in transit:
Emails Hacked In Transit
Using TLS is at least a starting point to help reduce this kind of thing. I am not well versed enough to know if it would prevent what the Chinse hackers did in this case to swap out email attachments in transit, however at least it provides authentication on both ends of the message and fixes a few problems in SSL.