Blue
Ravenous Bugblatter Beast of Traal
If you happen to get double effect, it feels cool. Nice thing.There's three possibilities: 0 effect, 1-action effect, or 2-action effect. In theory 1 should happen most often, with 0 or 2 happening less often and with an equal chance of either.
And though there's an ever-growing sense that casters expect their spells to work as intended (as in, always exactly hit their target area, always run their full course, etc.) every time, I don't buy that line of thought. Spells, particularly those cast under duress e.g. in almost any combat situation, should be able to fail, or miss (or hit the wrong target), or otherwise not quite go as intended; similar to someone shooting missiles into combat - most of the time it works but sometimes things go wrong.
Here, where there's an equal chance of getting more bang for your buck as there is of less, I fail to see any reason for complaint.
On the other hand, if you use your action and your spell slot and there's NO effect, you feel like naughty word. It is a greater negative than the bonus round. This is why lots of spells still do half damage on a save - because otherwise you are using your action and a limited resource for no effect.
Plus, your evaluation that they happen about the same is flawed. Combat in D&D is about dropping foes. That the foe will still be around for a second effect is a less likely than he was around for the first effect.
To sum up, the positive happens less and has less psychological effect, then the completely wasted effect.