I have to agree with you, it seems disappointing that external memory can't be used for more things.
A lot of that, though, depends on the software. For example, I wish Sygic would allow GPS maps to be stored more easily on the external card (and there are manual ways to force that), but they don't, as far as I know. I also wish games were better about allowing external storage. I'm not sure why more software vendors don't allow for storage on there. I suppose there is always that functionality trade off -- vendors worry that people will put critical data on a card, then forget, and remove the card at a crucial point while the software is running. Consider RAM on a PC -- if there were a way to eject it mid-operation, it would really make the operating system go haywire.
Same goes for some sort of automatic, seamless way of storing data there as overflow. That requires a LOT of smarts in the OS (not saying Android isn't smart). What if the external memory card starting getting used for system files at some point and you ejected the card mid-write? Again, haywire. Even high end OSes and embedded systems struggle with that sort of dynamic operation. It's hard to be both very flexible AND very dynamic.
Still, if you are careful to put everything that CAN go external on the SD card, you will be saving a lot of space:
Books (I got an eBook reader -- FBReader -- that would let me control where the books were stored, I don't think Play Books does -- again, disappointing)
and with a file browser or media server (I use BubblePNP and really like it), you'll be able to access those files wherever they might be, especially if the app in question lets you change the setting and it remembers.
-- Found this thread on another forum, some posts fall in line with what you are asking: Anyone else tired of the SD internal vs external discombobulation?
-- In the Camera app, make sure the little icon in the lower right corner is on the SD card-looking icon -- that is where it will store pics and movies. I had the issue where for some operations it was storing stuff internal, and some external. I believe you have to set that setting for each scenario: front camera, rear camera, pics, movies. (movies may always save to external if available, not sure).
-- When digital media (like for the camera) is stored, it will create a folder called "DCIM" on the cards. On my A100, the card mount points map to:
/mnt/sdcard (that's the internal)
/mnt/external_sd (that's the external)
The DCIM folders go from there, and mine has a "Camera" folder under that. You can also make other things point there, but only if the software vendor makes their software support that (like I mentioned previously).
-- If you are using a file explorer and can't see the /mnt directory, you may need to go into the browser settings and allow navigation to the top (root) level/ Some explorers lock that out and start you in the internal SD folder right off the bat. I set mine to always start me at "/" (root level) so I can go through "mnt" from there.
-- The browser will download files to the internal card in the folder "downloads". Not sure you can change this. I think other browsers, like Dolphin, let you control the download location. So, if you download a lot of large files from the internet, you might want to use a browser that lets you control that, again saving internal space.
Final thoughts: out of 5.4 GB of usable space on my alleged 8 GB device (not sure how much of the lost space is due to the OS and how much to uninstallable system apps that I could eliminate if I rooted), I have 700 MB of apps and about 1.3 GB of "Misc" on the internal card. That leaves 3.0 GB free still, almost 60% of the available space. I have quite a bit of stuff installed (around 50 apps), and that 1.3 GB of Misc is made up of Sygic maps and game content downloads. If I am careful to offload what can be offloaded, it is going to take a long time to run out of space unless I went crazy with games or needed to download 5-10 states worth of GPS all at once.
Hope these ideas help in some way...