Regular crushed coral is notoriously high in phosphate and is a bad choice, IMHO. Some people use it without issue, though. There was actually a few comparisons a while back, but I don't know if the numbers still hold true. ARM and Korallith were lower in PO4 compared to others, but were low in magnesium. The magnesium may or may not be of concern, though, depending on how it is depleted in your system.
Do you need one? No, not at all, unless you have an extremely large demand and/or a large tank. Up to a certain tank size and/or demand, it is actually cheaper to use a few other methods for most/all of your calcium demands. If you are keeping calcifying corals, the calcium in water changes won't be enough over time, but will likely be ok for soft corals. So yes, the addition of extra calcium and alkalinity(via any supplementation scheme--2 part, reactor, kalkwasser, etc.) will depend on what you have in your tank. That being said, there are people that are maintaining very large coral populations on 2 part and/or a kalkwasser alone. I've done it in the past, myself.