You can't really expect a mobile email client to allow third parties to add their own extensions to it.
If they were that fussed they could have tried to make a clone of a email client and integrate their own features. If it was better than the default client, people would use it (I use Sparrow on my iPhone for email, not touched the default Mail app for years)
You know how hard it is to get people to migrate from something as ingrained as an email app? Even a great client will suffer from the migration pain.
And to respond to criticisms of "plugin dev difficulty," that's bs. Browser clients have a world more complication when it comes to supporting a plugin environment, and they make it work just fine.
If they were that fussed they could have tried to make a clone of a email client and integrate their own features. If it was better than the default client, people would use it (I use Sparrow on my iPhone for email, not touched the default Mail app for years)