If you don't care if it's accurate and only use the speed readout as a relative indicator during the ride and between rides than GPS speed is "good enough." GPS speed is calculated from measured distances and time intervals and then averaged so it has inherent hysteresis. Most cyclists in my group don't use sensors. But if you want accuracy (and consistent, instantaneous accuracy), then you need a sensor. Just depends on how you ride and what you need in any given situation.

I have cadence and speed sensors, both Garmin, paired to my Edge.