Here's my take on Smart Previews - they are (almost) completely unnecessary for images that you're finished editing. Here's why I feel that way.
I keep only the last 4-6 months of work resident on my laptop, the rest is offline. I keep catalogs by year. When I import photos I build 1:1 previews, not smart previews, because all edits are local. I keep 1:1 previews forever, because I delete what I do not want. I have a system for rating and flagging images that allows me to identify images that are fully finished (published) as well as those I want to keep around to potentially revisit (keepers). When I archive them to offline storage I look at the published photos and decide whether or not I might need to produce a JPEG for sharing online again down the road, and on those I will create a Smart Preview as it will allow me to export a 1000px copy without accessing the offline storage. For those I will dump the 1:1 previews as they are unnecessary. I will also dump the 1:1 previews of all the keeper images since the only time I'll want to access it at that level will be when I want to do full edits, in which case I'll be sending the image to Photoshop and will need to access the remote storage anyway.
I suspect that if I traveled and kept most of my images offline, or used my iPad to do basic edits, Smart Previews might be a great alternative. But I see no need for them unless you want/need to do actual edits on your photos when storage is offline, and then only if you do all your editing in Lightroom (you could certainly do all your prep work remotely). 1:1 previews are all I need.