Couple of things: There is no good argument for completely-unchangeable forced-unmount. Just... isn't. Unmounting card at back open is like unmounting a USB stick when your hand nears the front of your desktop computer (or pick whatever orientation relative to your computer your hand will be to get to your desktop's USB ports. Sure, you may only be trying to pull out the USB device(s) and/or press the power switch. Or maybe you wanted to brush of some dust or your hand was there for computer-unrelated reasons.

Oh, and yes, you can change battery without shutting down - it's a major hack and requires tricking your N900 into thinking it's charging even when it isn't, but I think (memory's hazy) I've pulled that off with a combination of a tiny inch-sized micro-to-mini and micro-to-normal USB adapter (one or the other, not both at once) and USB host mode. Or just having access to an outlet and charger at which point you don't need to 'trick' it, it just does. But you do get about 30 seconds to hotswap the battery, which is more than enough time. Especially if you can open the battery cover BEFORE you switch batteries. (And no it doesn't matter if 5 users out there do this, or 50, or thousands. Some do, and giving people options shouldn't be limited to the very popular ones.)

More to the point though - ever drop the N900? Ever drop it without a case? If your back cover has NEVER managed to come off in the process, you have been very lucky, or very skilled at breaking the N900's fall by controlling its fall with your legs or other body parts.

I have dropped my N900 a rather large amount of times, and I have had the back cover fly off. Especially for users who use SD cards for swap (which is a lot more than hot-swap batteries, if you're going to ignore that demographic), a drop or other event (any event, like letting your kids fiddle with your device or something) that results in the back cover opening can be much less pleasant for usability than data corruption. Which, whether it's opening the back cover or just yanking your card out unmounted, is about equally likely if you're in the middle of writing data to your card, and similarly unlikely if nothing is writing to your microSD card.

If you aren't using your card for swap, force unmount is equally possibly annoying, but not a major deal. If you are using your card for swap, force unmount can be disastrous (more often just really annoying, disastrous is possible but less likely), while the vi_ proposed option allows for much more pleasant outcomes.