Please don't misunderstand me. Fade Step is a good ability. If it were not for the 8-slot limit, I would recommend it on every mage.
For me, a Rift Mage just doesn't have the room to fit it in. It's damage is sub-par compared to alternative choices. It's utility is no longer needed given Rift Mage skills. And it's mana-regen is wasted on any Rift Mage who knows how to keep weakened on a target.
1-4) I like my Rift Mage to have a focus ability and all three Rift Mage magic abilities (PotA, Stonefist, and veilstrike). I often am tempted to drop veilstrike, but it's quick CC utility and the ability to re-apply weakened is too hard to pass up. That's 4 abilities gone, and in just one tree.
5) Energy Barrage (one of the few single target mage abilities that can be tuned to swapping staff weapon types)
6) Firemine (too much synergy with PotA to not have it)
7) Immolate with upgrade toggle (can be spammed as a rift mage, i.e. too much synergy to pass up, plus you have to get it)
8) At this point, I am reminded that the Rift Mage is one of the few classes that can create cross-class combos all by themselves. I need something I can shatter. So Winter's Grasp comes naturally. Sometimes I swap it out for ice mine if I want to guarantee ice armor. But more likely I'll swap out immolate for ice mine (or blizzard).
Simply too many spells that have great synergy with Rift Mage, and Fade Step is not one of them.
That doesn't mean Fade Step sucks though. It's a great spell. In fact, I think it's a must-have before Skyhold. But after specialization, not so much (at least for a Rift Mage).