A toggle would be nice for power-users I suppose. I for one don't want my phone sucking battery heavily just to download an iMessage "Grumpy Cat" meme in the background 1ms faster. Honestly, even if I'm streaming Netflix, while downloading podcasts good LTE signal will be MORE than adequate.
Where I'd want to "smart" data mode to kick in is if I trigger a large download for offline use, or am doing something else that could really benefit from 5G.