The reason for this that Mail Droid was already in Google Play when I started working on my app.... So its developer has had a lot of more time to do "stuff", and it shows.
Android 4 has a system spell checker, and finally passed 2.3 this week as the most popular Android version. As time goes on, this will only deepen.
It should be noted that, just like with soft keyboards, it's possible for third party apps to offer system-wide spell check services.
This leaves Android 2.* -- and yes, I just don't have the time to write one, so decided to leverage someone else's work, which he very kindly made available as a separate component.