I realize I'm probably late to the party, but I've only recently been spending more time in SW2018 (some customers upgrade slowly), and I've come across parts imported from STEP files apparently using this 'feature'. I give it a thumbs down due to the performance hit. I couldn't figure out why a medium sized assembly (created by a colleague) was running so slowly. It seems like it's rebuilding the part from the STEP file every time you rebuild your model. Also every time you open it. I found several of these in a the assembly. The other downside for me, is that I have another external reference that must be imported into my vault. I don't plan on updating 99.9% of my STEP files. They are mostly COTS parts, so this is an un-appreciated feature for me. Maybe this is useful for other actively updated file types, i.e. if a vendor was customizing a component in a different CAD package as you were using it in your design, but I don't have much use for it at this time.
Fortunately,it's easy to eliminate. Just right-click the feature and dissolve it, just like a library feature. As soon as it's done, you see the imported bodies we're used to, and the rebuild time is nearly zero as expected. My next step is always to save as part to eliminate opening lots of files with 1 imported solid each in them. Also trims number of files going into the PDM vault. The last step (if you want to get extreme) is joining bodies and defeaturing internal stuff.
You can also disable this feature if you don't want to go through the steps of dissolving it. Go to File->Open, change filetype to STEP -> General, and the first checkbox at the top is 'Enable 3D Interconnect'.