Clearly this is a pet idea for more people than most, given how often it has come up.
But flicking through the blog post I disagree that this should be a number one priority. The majority of the features discussed focus on two things - improving the usability of existing features (eg spending and targets which are essentially broken until incoming payments can be categorised and month start altered) or making Monzo a more sustainable business.
I think a lot of people would be disappointed if a more complex feature like Joint accounts were priotised over improving basic features that are used by the majority of users.