There are cameras out there so small they're nearly credit-card sized, and they _still_ manage to use a full-size, proper, external SD slot. So I don't buy into that.
The real point is that an internal micro-SD is useless for easy access to cards from other devices, even if you went ahead and replaced every card of those devices with micro-cards in adapters. An internal microSD card is what's used for a traditional phone with little or no internal storage ("memory" to some), as a permant, never-removed storage extension. The N900, with its 32GB built-in storage, hardly needs that.

The N900 is a computer, a media central, a lot of things. An external SD slot makes it possible to easily insert cards from my other devices: Cameras, Zoom H4 4-track recorder, PDAs, Video, soon an R16 recorder, and more.

Get rid of that internal microSD, it's not very useful. Install something useful instead: full-size, external SD slot (and make it SDXC compatible when you're at it, as suggested by soeiro).