That would be not-so-fun surprise when doing a renovation. I know lots of DIYers who like to cut out the copper segments and redo with plastic since it's easier to work with.

This has killed a few plumbers before. Not due to using it as a ground which is honestly completely safe but due to a faulty water heater that was inadvertently using the copper pipe as the neutral. The plumber turns off the water and cuts the pipe, then grabs both ends and pulls them apart and now he's got wet hands on both sides of the circuit he just cut.

DIY? The whole industry uses it and many cities use plastic pipes to deliver water as well. Simply put, you're not supposed to use water pipes for ground. That was done many years ago (prior to 1980 here).