Actually people without mental imagery do well on tasks that involve mental imagery: http://discovermagazine.com/2010/mar/23-the-brain-look-deep-into-minds-eye

yet he could do lots of things that would seem impossible without one. Without any effort he could give the scientists detailed descriptions of landmarks around Edinburgh, for example. He could remember visual details, but he couldn’t “see” them. Della Sala and Zeman asked MX to say whether each letter of the alphabet had a low-hanging tail (like g and j). He got every one right. They asked him about specific details of the faces of famous people (“Does Tony Blair have light-colored eyes?”). He did just as well as the architects.

...showed MX pairs of pictures, each one consisting of an object made up of 10 cubes. MX had to say whether the pairs of objects were different things or actually the same thing shown from two different perspectives. Normal people solve this puzzle in a strikingly consistent way, with their response time depending on how much the angle of perspective differs between the two objects: The bigger the difference, the longer it takes people to decide whether the objects are the same. Some psychologists have interpreted this pattern of results to mean that we really do need the mind’s eye in order to solve some kinds of problems. When deciding if two 3-D objects are the same, we have to mentally rotate them. The larger the angle between the two, the longer it takes to rotate them and come up with the answer. If we used some other kind of reasoning, it would be surprising to see such a reliable link between the difference in perspective and the time it takes us to solve the puzzle.

MX’s results flew in the face of that explanation. When he solved the puzzles, he always took about the same amount of time to answer—and he got every one right.