I bought two of those motors about 4 years ago. They are made by Hsiang Neng (Taiwanese company), and are just one step above 'el-cheapo.' A cap probably wont do much, but wont hurt. Id recommend putting one on.
What I hated about these motors is the mounting screw holes in the front (as you can see in the picture). The problem with the holes is that if you tightened a screw too tight in them, the motor jammed up and stopped working. I have no idea why they jam, but they do. This meant whatever I mounted to them had to be loose . . . I lost many a screw with these motors . . .
What I do like about them is that they are powerful and cheap.