If you think about it, fish in the wild are subjected to 100% water changes constantly - that is unless they live in stagnant or very slow moving pools.
The biggest danger when performing a partial water change, as I understand it, is inadvertantly causing a massive change in temperature of pH. It is also possible to forget to add dechlorinator - I've done that *grins sheepishly*.
I think that these are the reasons you never see anyone advocating a greater than 50% water change per day - if you goof up during the water change, your mistake would be diluted by at least a factor of two.
Ultimately, the minimum frequency and extent of water changes depends on the type of fish you have, the size of the tank, and the quality of your tap water.
That said, adding fresh water to any sized tank , regardless of its occupants, is like opening a window in a stuffy room - the fish love it and it is probably the best health tonic you can give them.
If I possessed the motivation, I would do 20% water changes every day (I'd also work out).
The reality is that I do a 40% water change once or twice a week (and I don't work out at all).
So, as long as you don't exceed 50% a day, I don't think it's possible to do too many water changes.