Well, it's the most widely spoken international language, spoken in over a hundred countries, by an estimated 2-5M people. There's a rich literature (probably 30-50K books), vibrant music scene, and support in open source software (Linux, Firefox, Google products) is usually pretty good.
But the issue is not how widely Esperanto, or any other language, is spoken. If you assume that languages should only be supported according to their number of speakers, you leave no room for useful languages, bridge languages, auxiliary languages, or growing languages. Even if Esperanto had only 100 speakers, it'd be worthwhile to support, if it's easy to learn, and easy for non-speakers to understand.
It's not a "severe burden" to consider non-ASCII letters as letters. Unicode is pretty straightforward to work with, and if you want to support more than just English, it's a necessity. There's no need to have a "priority list" of letters you consider more or less important than others. That attitude comes across as very Anglocentric.