[toni-a]

I understand equivalence quite well, and I totally agree with you that full frame can indeed replace APS-C
I was talking about the practical part
let me explain with photos
This is David-a with 85mmf1.8 on 5D



This is David-a with 85mmf1.8 on 7D2
both taken at MFD
of course you can crop 5D image and get the same result yes, but it's practical with 7D2
suppose you are shooting macro, you can see the obvious advantage, be it only in resolution 
That's why I recommended a macro lens rather than a fast lens if you want dual system like me.
For me I usually carry only EOSRP however during events, to avoid swapping lenses i carry also 7D2