Keeping prepaid card for those who want both

What additional security does keeping them together provide you? If you’re talking about Monzo’s “Location based security”, I don’t think having your phone and card in different places will stop you being able to use your card. The information that’s been provided so far indicates that the location of your phone is just one of the signals used to detect fraudulent activity, and it will be more about the pattern of where your phone and card are, not strictly whether they are within a few feet of each other.

As a point of reference, I’ve never even turned on location-based security, and have not had any problems.