I would like to add one thing to Jess' post.
If you recast it too late, and it falls off before you finish casting it, your next spell casts without 15% haste. If your UA refresh lands before Soul Fire does (it can happen during eradication), then UA doesn't have 15% haste.It's frustrating. Knowing how important it is makes it stressful when it drops. Do you really care when any of your DoTs drop or miss? No, you recast and get on with it. This leaves you scrabbling for Soul Burn or leaves you stood in a stupid place for three seconds while you desperately try to rectify it.
Lets say you hit on average for 10k shadow bolts, including crits, with a cast time of 2 seconds without ISF, this would be 1.74s with ISF up. If you launch a Soul Fire late and don't want to refresh your dot so cast SB instead, you lose .26s or ~750 damage in this scenario, not to mention UA falling off and prioritizing a low DPCT spell over a high one.
I hate it for the simple fact that I have to completely change my priority based on whether its going to land in time or not.