I'm not really sure what the advantage of these lenses is, to be honest. They do protect the LEDs from direct contact, but they don't make the boards water resistant or anything like that. They're probably a good dust cover, and they do concentrate more light over a smaller area with the 90-degree focus, but I'd argue you can do that yourself by lowering the board.
One reason I'm not a big fan of plastic lenses is because they absorb light. And once they get dirty, they absorb even more. So unless you're out there cleaning them on a regular basis, you may find your light output diminishes over time. It kinda defeats the purpose of running a super-efficient board IMO.
One area where these types of lenses do have an advantage is when they fully seal the unit, making them water and dust proof (or at least resistant) in greenhouses. But in this case, they don't help seal the whole unit - the connectors are still exposed.
If someone else can offer any other advantages, I'd also like to know.