The #1 rule is always to find as light tools as possible to reduce the risk of work related disorders linked to the usage of power tools. Weight is one of several factors that makes up the overall ergonomics of a power tool.
There are only a few occasions when a heavier tool has a positive effect on the ergonomics of the user: grinding and chipping on horizontal surfaces are two examples of that.
The benefits with heavy tools are lower vibration emission and, on occasion, less need to apply feed force.
The disadvantage is the increased risk of work-related musculoskeletal disorders due to the forces the weight applies to the hand-arm-shoulder system.
For all jobs where no particular feed force is required, the tool weight should be kept to a minimum. If the job also includes bad working postures and a lot of repetitive motions it’s even more important to make sure that the tool is light.
Weight is like any other external force and it should be kept as low as possible at all times.
Material removal tools require a certain amount of feed force to remove the material.