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An SCR is normally fired using a small gate voltage so that you actually control something.If you went about triggering SCR's by using its Forward Breakover voltage then their is no point in putting them in there for any reason since you dont have any control.
You may be confusing an SCR with a 4-layer diode (The Four-Layer Diode) which is similar in construction to an SCR but does not have a gate. It is latched on by exceeding it's forward breakover voltage. SCRs can not be reliably used in that mode.