Rain areas are more stable within the Rain-Thunder display but rain can move quickly, or else just start and stop randomly.
But with regard to thunderstorms, they are short-term events and their occurrence is often very patchy and local. So they'll rarely 'move' smoothly as what you see displayed is a series of separate storms that are forecast to form and decaying in less than two hours (usually).
But the model moves foward in three-hourly steps. So it's unlikely to become smoother until the model's time steps are also about 1-hour long. The AROME model has 1-hour steps now but its coverage is very limited.
The other problem is that the forecast location and formation of any thunderstorm can not be very accurately predicted by a model. The formation process is more chaotic than a model can reliably predict. What you therefore see within the display is only giving you an idea of how likely a storm is within that area and within that time frame. And to indicate the likely storm intensity.
Models can forecast the likely thunderstorm intensity, if a storm does form in an area within a given time. But a model can not tell you accurately if one will form or where.
So even if the display was made smoother, it still would not be reliable, it's just a guide to what's likely to occur in the area if a storm forms.