...It stands to reason that the app you open the document in should be the app you send the document from. But thats against many peoples work style. AirDrop needs to work between Mac and iOS....
...Personally, I prefer sending web links to documents in Dropbox or Box so the Mail attachments has never been an issue.
I guess I'm one of the 'many people',
Dryland.
I don't (and I can't think of anybody I know) who, having a document on file, first open it in Word or Pages or whatever and then proceed to create an email from there. Similarly, having got an email from someone that requires a reply best including an attachment, I don't expect to have to cut the original email, open Word / Pages > Send to Mail > Paste original email and then respond to it.
Recognizing I wish to send an email, I open whatever Mail client I have, create the email and attach the appropriate file from there. Similarly, if I want to reply to a received email, I do so from within the mail program and attach the supporting file. That's the way 'many people' expect email to function.
If it seems impossible to do with a locally stored file in a mobile device, I'm okay with accessing a cloud service to find a copy of that file to attach it. Really. Providing it'd not going to cost me anything (read: data use when out of the country - say, on business). But again, that's gotta be done through a Mail app.
As for attaching links to files online - I get what you are saying, but a lot of people / companies / corporations are hesitant about being forced to go online to retrieve files (security and all), and it's poor form to require them to access something of yours that you have sent. An email should arrive complete, just like a 'real' letter - not expecting someone to then go to post office to get the rest of your correspondence.
If Apple can't (won't) step up, they shouldn't be surprised if the usefulness of their app (and iCloud) comes into question. Their success rate with built-in apps and cloud storage (over 3rd Party) is already pretty tenuous - do they really want to let another one of their apps to be tossed into a 'Junk Drawer' folder in iOS?