This question keeps coming up time and time again. To my knowledge there is no good reason for the “apps” to only use phpfina.
There used to be a way of editing the code so that other feed engines could also be used without any problems, it has been a while since I tried it so I don’t know if it still works or not. See Does emoncms expects data to be send reguarly? - #2 by pb66 for more info, however, all the links to the github code seem to have moved around due to the code being refactored over time. So it maybe pretty difficult to work out.
Perhaps when i get time I will try and redo it and write it up better as there doesn’t seem to be any plans to change the way it works any time soon.
Yes keeping a second set of feeds just for displaying the same data in the apps is a waste of both disk space and cpu load, but more importantly it could shorten the sdcard life due to the increased writes on a local emonPi/Base. On emoncms.org we also get charged for the additional feeds. This basically equates to an additional charge just for using the apps pages.
[edit] although the github links may not make sense, a quick search of the app repo for "engine":"5" should help locate where the changes need to be made