Running desktop apps is where making the X10iii external monitor work comes in.
I can’t usefully do much work on a phone screen, but if I could plug a large monitor into the phone, it is certainly more than capable of a lot of what my linux desktop does.
On the N900, it was a pretty useful desktop computer with VNC for writing documents and email. More graphical things and VNC became a bit painful - perhaps it would be a lot better now, but I would really prefer just to have a monitor plug in directly.
My N900 was also formatted as a Suse bootdisk i.e. you could plug it into a PC, and it would usb-boot into a full desktop Linux. This was amusing when I had it plugged into a aircraft entertainment usb for charging, and when they rebooted the entertainment system half way through the flight (quite a common occurence), my seat-back display rebooted into Linux. Oops.