Xrade isn't quite right. The rubies actually reduce the duration of all of your DoTs by 15/30% to increase their damage by 30/60%. Depending on the DoT that you apply, this boost may matter or it may not. For the DoTs applied by grenades, where the damage ticks every 2s and you have a relatively long cooldown on their use, these rubies will only boost your overall damage by ~6.67%, just with larger individual ticks. That's valuable for PvP, but otherwise fairly meh. More important, though, is that some of the other DoTs you apply can be reapplied near-constantly if you're in Assault spec. Background Radiation, for instance, can be basically kept up forever and therefore essentially gets a 60% boost in damage because of these rubies.
However. All of this presumes you are able to reapply the DoT within a certain amount of time. There are builds where you pull points away from Swiftness to push into other stats. In those cases, you may very well only gain the minimum 6.67% increase from these rubies. That is still a reasonable boost, but depending on how many rubies you have available to invest, you could gain more benefit from placing them elsewhere. If you're running Assault spec in PvP, though, they become MUCH more valuable as burst damage matters a lot in that environment.