Probably for the 99 % of the time a single throttle will be perfectly fine, so buy another throttle is not needed for somebody who already owns one. But the dual throttle obviously have its uses apart from the engine start, like when you get a compressor stall and need to reduce the throttle of a single engine.. or when the plane is damaged and need to apply more thrust in one engine to fly at level..
This Monday I was flying a training mission with some mates and was flying the F-5.. never had many problems with their engines, but this time my right engine was very prone to fail and get a lot of compressor stalls and strange readings on the gauges and the dual throttle was needed a lot of times. Anyway, these issues are very strange if they aren't planned with the mission editor, so the dual throttle is not indispensable.