I've got a Strida 5.2 and absolutely love the greaseless experience.
For ordinary chain bikes, when could a Biologic FreeDrive Chain Cover be used without causing troubles, but only provide a cleaner ride?
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I'd say "when there's no rain, mud or grit on your route". That thing looks as though it's designed to collect any loose material that falls into the open lower section and make sure it stays on your chain forever.
Recumbents commonly use sections of poly pipe (cheap black flexible plastic pipe) to cover their very long exposed sections of chain, and that works fairly well. You could do the same on the shorter chain of an upright and get most of the benefit of the solution above without the penalty. Easier with a singlespeed or hub geared bike, but possible with derailleurs (the attachments need to be flexible).