In reply to DonnStar, and adding some more thoughts (for clarity)...
Firstly I'll list the steps again for the benefit of others looking for the solution, who don't skip back a page.
GOODREADER
1. in your MAIL app, do a reply, then select all and copy.
2. Open GoodReader.
3. Find the file you'd like to attach - it doesn't have to be in GoodReader, so long as you can "open-in" and move it there.
4. In GoodReader, tap on manage files and select the file(s) you want to attach, then tap on "email".
5. Paste the entire "reply to" into the email that now includes your attachment(s).
Yeah, it's a workaround but it works very well. By now I'm just used to swapping in and out of apps to get a decent workflow.
I'm not trying to defend Apple or any other corporation/developer's ethics, however if you think about it, there's a zillion "busine$$" reasons why we have the situation we have. Apple don't make the apps, they make the hardware and the OS - and they control the business relationship with app developers (they didn't always. You'd need to dig a little history – look at the kind of thing Apple were doing around the time of "openDoc" and the dramatic change in their direction c.1996-8). I knew something like the iPad was coming - I hoped and to my dismay iPhone came first, of course it makes sense from a business p.o.v. We are probably not the typical user that Apple based their projections on.
On the other hand the app developers have seized the potential – and I guess they're the ones we should be lobbying for the solutions wanted. Whether we get all of them right now - who knows. There are undoubtedly just as many clever solutions locked up in a dark cupboard as there are the clever (and frustrated) people who create them. They are stymied by the finance manager who says, "we'll roll out 10% of your solution every 2 months as bug fixes, updates and in-app purchases!"
I wouldn't be surprised if before too long, we won't be seeing very many "off-line" improvements, as more apps become subscription based tools and services and go into the cloud.