I think it is all about perspective. Different tools have different uses. Although I have never used the Shake Weight, from my perspective it seems to be like isometric vibration training.
Depending upon one’s level of fitness, the Shake Weight might be the appropriate instrument to use with a specific client.
A client might need to gradually increase strength in a limited range of motion or at a specific joint angle. It could be an effective product for the wheel-chair bound, elderly, nursing home bound client. Personally, I can see how it can be an effective tool.
Without knowing what the client needs, I am not in a position to castigate the product.