I've never heard of any problems. I believe it was purposely designed the way it was to give repairmen easy access to be able to see the machine in operation. I don't think there is any extra stress on the switch itself than when the lid is closed, and I don't see any way it would do damage to the machine itself.
Even if it could somehow deform the switch mechanism over time, I think that would just mean that the switch would remain engaged all the time even without the clothespin. And I really don't think there is enough pressure on it to do that. But just in case, and I just checked, you can get replacement switch assemblies (part number 204791) on the internet for around $20, assuming you couldn't just carefully bend the mechanism back into shape.
Plus there would be less wear on the switch itself leaving it permanently engaged than constantly engaging and disengaging it.
Being exceptionally cheap, I didn't even waste a clothespin on mine, I cut the end off of an old radiator hose and squeezed it in above the switch.