No immediate confidentiality is achievable by using a trusted mail server while interacting with untrusted ones, but doing what seems right often works as an improvement. I find it analogous to, among other things, not throwing garbage on the ground: by carrying it to a trash bin, you don't make all streets clean at once, and it may be an inconvenience, but once more people do it, streets do become cleaner. And even if others keep just dumping trash on the ground, it may be nice to know that you don't contribute to the unpleasantness. Likewise with a multitude of other areas, including good practices in programming and administration, good manners: it is unfortunate that not everyone follows them, offsetting the effort put by those who do, but it is not necessarily a reason to give up.