I can see the decompression lever causing fuel issues. All that nasty (and diesel is a nasty fuel) has to go somewhere when you hit that level... and it's under a lot of pressure too, so it is going to go -everywhere-
But I cannot see how it can hurt the exhaust valves. You spin the engine over with the lever open to start, why would it hurt the valves to open it when running?
Well, I'm just guessing here, and I'm sure that it depends on the particular engine, but the decompression lever probably holds the exhaust valves open (so that the piston doesn't generate any compression when it strokes upwards, pushing the air and fuel mixture out), and so if you actuate it when the engine is running at speed then it could either A) cause the valve to hit the piston or B) strip out whatever gear/sprocket that is used to actuate the valve to decompress. When you are starting the engine (especially by hand) you are only cranking a few RPMs, when it is running (even at idle) you are at least 500 RPMs.