Reason why i would like php 7.1 uploaded to apt.wm.org is because it's performance is much better. I would like to experiment with php 7.1 more with phabricator. When i have finished that testing i would like to eventually upgrade the phabricator server (prod) to 7.1.
Reason why 7.1 and not 7.0 is because 7.0 removed a feature phabricator was using and was only recently added back in 7.1 + it always is better to go with a .1 release then a .0 release.
Also stretch is using 7.0 so thats why we need it also uploaded to apt.wm.org.