Chaos bolt scales with crit, and under 20% the extra crit chance does a lot to make sure your shadowburns crit. With T15 2 set you also can fit in an extra CB/SB during dark soul.
Using embers on trinket procs is good and you should do this, but you should ALSO pool up embers for dark soul. Spend your embers on both. Furthermore, you're likely to get a trinket proc during dark soul anyway.
Unless you struggle to generate 4 full embers in between dark souls, you should use trinket procs as windows to dump at least an ember or two, depending on the proc.