I know no system is perfect, but if it is so bad, why is the "Compatible up to:" field even posted on the plugin pages?
Because it's not bad. It's just ...
None of the data you get from a plugin, be it "Compatible up to:" or rating or popularity or even support requests that are resolved tell you the whole story.
Look at A Sunday Afternoon on the Island of La Grande Jatte. From afar, it's simple. A painting of people on the island. But if you walk up to the painting (it lives in Chicago, I love visiting it) you'll see Seurat painted entirely by dots! As you step in and out, the painting changes and your perspective and understanding of the work as a whole changes. You cannot simply say 'There is blue paint.' and make your final decision that this painting will look nice against a blue wall. You have to consider how it will look up close, far away, and will it be better to blend or contrast. Seurat liked the contrast, which is why it has a brown border and a white frame.
With your plugin review, you must consider the whole picture. Who wrote it? What does their personal/plugin site look like? How often is it updated? Is it well documented? What problems have people had? Are those support posts resolved? What's in the Compatibility Matrix? How many downloads? What's the popularity? What's the star rating?
You can't just take one and say 'This is compatible with 3.4, therefore it is superior!' I wish you could :)