Plugin development: is adding empty index.php files necessary?

It is a good practice to include index.php, with and without a wordpress context. Appach 2.4+ IIRC will add a log message about a missing file if someone access the directory itself, which makes reading the error log more annoying.

As for the reason you quote, obviously this do not have any security related impact, and the “wordpress will have to parse one more file once a year”…. you are a great developer if that is the only performance issue you have in your code, and with php 5.6 code caching even that claim might not be true any longer.

You can put your code in the index.php file but by now people have expectations about the naming of the “main” plugin file, and therefor it might create pointless confusion as most people will assume index.php is an empty file