You need to put things into perspective. If you reuse these switches and properly discard them at the end of their life there is nothing wrong with them.
When ROHS alternatives exist, then why not use them instead?
The amount of Hg that humans have already spread in the environment and continue to spread is enormous. You contribute to that simply by being alive and using industrial products.
I know the situation seems bleak, but in saying this you are discounting the environmental hazard, and also the idea that people have some volition to change the situation. Sure, the world is already screwed, so why not buy some more mercury
The fact is that there is a significant amount of mercury in a mercury switch (I just looked this up, and there is about 50-80 times as much mercury in one automotive trunk mercury swich as there is in a fluorescent light bulb), and it concentrates in fish.
So hey, why not use something that is less harmful?