Yes, I always put it in airplane mode in the past for the reasons you mentioned. But it was never enough for them, so I just learned to hide it. I really hate rules that are created and enforced out of ignorance.
It's not ignorance. Rules about electronic devices were created by the FAA, based on sound technical reasons. Another poster provided a link to an IEEE article that explains the problem very well, although it may not be comprehensible to many people.
It should be noted that the FAA doesn't actually prohibit electronic devices. The rules makes the aircraft operator responsible for determining whether an electronic device interferes, and allowing the device only if they are sure it doesn't. So, in an abundance of caution, the aircraft operators required all devices not under their control to be turned off.
Electronic equipment installed on an airplane is carefully tested to be sure that it doesn't interfere with the avionics. The TSO standards (for aircraft) are much stricter than FCC type acceptance (for your cell phone).
All electronic devices (including your computer, even if it has no wireless capability) are required to have FCC Type acceptance. Do you know why? It's because your computer is transmitting all the time, just by being turned on. Any digital electronic device does this -- it's a law of physics. There's at least one oscillator in the device that generates a "clock" to drive the components of the device, like the microprocessor. The transmission is relatively weak, and the device can reduce transmissions with shielding, but it's impossible to completely eliminate them without putting the device in a metal box, with no holes for a headset, power, etc.
So what changed? There have been significant advances in electronic device design since the rules were first implemented in the 60's (or maybe earlier). Your phone, computer, etc. are better shielded. And aircraft avionics have better selectivity (a term describing a radio's ability to reject other signals). But again, there are laws of physics (and practical cost and weight issues) that prevent a radio from completely rejecting unwanted signals.
Maybe the flight attendant doesn't understand all the issues. But, he/she is required to enforce FAA rules, at the risk of discipline by their employer. And if her employer doesn't instruct her to enforce these rules, the airline can be fined or sanctioned by the FAA. So, if you have a problem with the rules, take it up with the FAA.