If UBports allowed me to use my phone as a desktop (with normal Linux desktop apps e.g. Emacs working as expected) I'd absolutely do it! Sadly my current device is a OnePlus One which doesn't support external display connectivity... it'd probably also be more useful with newer, more powerful hardware as well. The OPO is a 5 year old phone now, phone hardware has come a long way since then!
My personal use case is as a sysadmin; I carry a laptop with me to and from work, and if I want to work on a personal machine I have to bring a second laptop with me. My phone could replace the second laptop if I left a dock at the office. Thanks to Emacs's TRAMP functionality, I can connect to a remote system to work on things that require more computational power than the phone by itself can provide.