Evolution’s goals (camel) are definitely different from Tinymail’s goal (camel-lite). You definitely can’t substitute each other in all places. Users wont be happy to use a email client that doesn’t even support message threading on a desktop. Philip posted an overview camel-lite onto the evolution-hackers list some time back. Camel-lite has a lot of things that can be merged with upstream camel like pop summary, some IMAP improvements (non-hacky stuffs). I’m sure that the discussion is ON between Varadhan and Philip to get the pop summary into trunk for early 2.14 cycle and more merging of useful things down the time. But I won’t / don’t think Evolution would move away from camel to camel-lite any time.
Update: I just saw that Sankar (who maintains EPlugins) has posted a nice blog about Evolution/Camel/Camel-lite. Though I don’t agree to all the points, some interesting differences and goals.