I'm saying that if you delete it from your iPhone and iTunes and it's not in the App Store, you're out of luck unless you have a backup. I have that 12 Days app too but if I remove it from the phone and iTunes, there is no where to re-download it from.
thats not true.
but you should still backup everything to itunes anyway like you said
if i delete "flappy bird" or "ultimate warrior" from my iPhone, i do not need itunes to get it. all you do is :
grab iphone
click on app store
click on updates
click on purchased
there are two tabs
one is ALL and one is Not on this iPhone
click on Not on this iPhone
every single purchase you ever made is on that page. even if its removed. there is still a download link
it has always been this way. (unless someone sued apple!!!! lol)
for the heck of it i turned off wifi to make sure i wasn't connected to itunes and repeated the same steps and sure enough i was still able to download "iPhone 4 case program" from apple.
there might be some stuff like books or media that you can no longer get, once they are removed from the store. but as far as apps go, once you pay for the app even $0.00 that app belongs to you and you can always get your app from the app store.
if the app cannot be retrieved from the app store because the link will not show up, it is most likely until proven otherwise , that it is not compatible with your device.
for example.
flashlight by ihandy will not show up on an iPad, because the iPad does not have a light
here is a screen cap of my iPhone 6+ running the iPhone 4 case program. i did not use itunes to restore my purchase.