I just received a new Rainbow this week and can confirm it has hooks for a liner. Both the Rainbow and Double Rainbow have the same length cross strut, and are the same length, so it's possible one liner sold is usable for both tents. A quick call out to Henry would clear this up.
The problem I see with comparing it to the double wall options is that any drips that fall on the liner will fall onto the inside of the mesh wall, not the outside. Thus it won't be quite as protective. It will, however, protect the area directly over your sleeping bag. I suppose one could permanently sew the liner onto the mesh, but I'm not sure that would make much difference. The Rainbow does have a flat roof, though, so I suppose it is more likely to drip condensation vs. pyramid-shaped tents.
When you consider that the Vapr NANO bivy sack sold on this sight only weight 4.6oz, and would provide better drip and wind protection, it's hard to justify the weight of the liner for solo use. Of course, there's a big price difference between the Vapr NANO and the liner, but it would be quite easy to sew an all-Pertex Quantum overbag for quite cheap.