Markerlights are super useful in 8th, as they last the whole turn and are not removed when a single unit uses them.
This is, perhaps, a modest exaggeration. The good news about the new markerlights is that the marked unit stays marked no matter how many people shoot at them. The bad news is, for the vast majority of the time, the only thing your units are gaining is a measly 8% accuracy bump unless you stack up on seeker missiles (which still miss about half the time) or are one of the few units packing weapons with the heavy designation who might want to move (Hammerheads and Broadsides, basically).
That said, using Tetras means the odds of you getting to the +1 BS level of hits is much, much better. That becomes a 27% accuracy bump, which is great. I'll have to consider these guys closely.