I think if you’re already using core and it works for you, you should just keep using core. It will do everything that this smartapp does and a lot more.
But the challenge with core is that it’s really technical. Just trying to explain to somebody what a “latching piston” is will tell you immediately whether they have a programming/technical background or not. 
There are quite a few people who have tried to use core and ended up deciding that they will only use it when someone in the core peer assistance thread gives them the exact piston they need for a particular use case. Which is fine, there are a lot of people who are glad to help, but it is a little unsettling.
So I think this smartapp creates another option, and why not? I particularly like the visual presentation, because I think that’s one of the areas which is really daunting to non-technical users of core.
Choice is good. 
BTW, those who want to combine multiple sensors into a zone can use @mike_maxwell’s zone manager and then just put the zone manager master sensor as the trigger in this Smartapp.