Side visibility is important, as you noticed already.
The Cateye Rapid offers that too, now in its 3rd generation (Rapid X3) with full side beam and beam towards the seat.
that is an interesting change in design philosophy. My current Cateye taillights have a fairly narrow beam that requires getting the light position set up with some precision in order to have the beam directed horizontally and towards the driver's eyes. This design goes the opposite direction, radiating light in all directions (at least in azimuth) and offering no adjustment for vertical angles.
Radiating light over such a wide angle is going to reduce the amount of light reaching a particular area. I wonder how bright it will look when compared to an earlier type of design that uses a narrower beam?
I also notice that this uses USB charging. Personally, I'm not sure that I like this trend. It appears to encourage the use of non-replaceable lithium batteries. Cateye is very good about making spare parts available, and they don't list a replacement battery.
On the plus side... Cateye still lists the TL-LD630-R, which is my preferred taillight and powered by a AA cell.