I too have done this, for years. Perhaps I just don't know any better. My road bike is 105-equipped, and has not suffered in the least from... tandem shifting? Syncro-shifting? Whatever.
I can tell you this: If your bike's drivetrain is not clean, properly adjusted, and well lubed, you will find yourself flinging the chain off of the chainrings before too long. This only happenned to me once, and it was my bad... I had ridden in the rain earlier in the day, and by late afternoon, we were somewhat less than a well-oiled machine.
I have not noticed any other wear or damage, and this bike has some 17K miles on it.
I do this also on my hybrid... it is equipped with older RX100 parts, and circa-1989 Rapidfire shifters. Works like a charm, up or down through the ratios.
Also... older Campagnolo gruppos really hate to be treated like that. They seem to prefer slow, deliberate shifts.