I think I would prefer if the GPS was given the boot and auto changed to NAV when an ILS frequency was selected... AS LONG AS THERE WAS A BIG BRIGHT LAMP FLASHING TO ALERT ME TO THE FACT!
Was this requirement waived in the early GPS systems?
Cockpit annunciation was a required part of the installation for IFR certification, and was very much a requirement for early GPS. Bear in mind that early GPS were enroute and terminal only, with VHF or other navigation required for approach.
Integration with FMS in which the user doesn't discriminate between inputs (but allows the box to do this) have largely done away with some of the more cumbersome procedures that were formerly part of flying an RNAV or GPS approach. With most current units, there's no need to manually sequence waypoints, or veryify each waypoint outside of the database date, etc.
Some systems allow nearly everything to be selected through the FMS (or comparable unit) while others still require manual selection/deselection and tuning. Knowing your specific installation is always critical to safety.
Switching errors have always been a concern, and may always be a concern. Add this to one more good reason to follow a detailed checklist. You're not the only one to inadvertantly follow the wrong signal. I've done it too...I even did it during a sim ride during an interview once...tuned the VOR instead of the ILS...and then even tried to follow it. Very embarassing...but fortunately in the safety of a sim. Mistakes happen. Just make sure that the lessons that come with them are survivable, and stick.
Get to know your radios and annunciation, and develop a technique for verifying what's in use on every approach. Back up your approaches with GPS/FMS, and always verify that what you're seeing is what you should be seeing. Tuning and identifying isn't just listening to the morse, but verifying that everything in the cockpit is set up properly, and configured for the procedure in use.