The electronics do send out weak signals. An entire plane full of electronic devices may well cause problems. It’s not an issue of ‘restricted wavelengths’, static is put out by the devices in a broad band, because it’s just white noise, not a directed signal. In flight this is less of an issue, because they have much less of an issue of running into something that high up, or losing their way since the airplane’s own electronics are pretty great. But when landing and taking off, the information is much, much more crucial and so they want to take no chances. Mid-air collisions are horrific.
Also, they still do restrict wireless devices in mid-flight, although this is changing (probably with better tech and shielding). WiFi is being used more, but cell-phone signals, which work on a different radio band (iirc), are still restricted during flight. You can turn on your phone’s wifi, but not your phone’s cell phone capabilities. I don’t know about stuff before airplane mode was around, but they do care now. You can’t have a phone on in normal mode during flight. Also, before airplane mode was around there were probably not enough cell phones around to be of consequence.
And yes, a single phone turned on will probably do nothing of consequences. But if everyone in the plane said ‘fuck it, it’s totally not a problem, I have to check my email!” you’re probably going to have an issue.
Also, I just took a flight a few weeks ago, so this is recent info.
Also also, as a guy who flies quite frequently (usually about 6–8 times a year), i can tell you it’s incredibly annoying to see someone fiddling on their phone during takeoff and landing. Seriously, it’s like 20 minutes. Turn the damn thing off.