Originally Posted by sdsearch

For several reasons:

1. The effect can be cumulative. So one person by themselves leaving a device on that should be off may not matter, but many people on the plane may matter.

2. Taking off and safely landing doesn't mean there werent' issues (that good pilots managed to work around). You may be causing issues that are unreported and cause some problems, even if they don't escalate to an obvious-to-passengers problem with take-off or landing.

3. A properly functioning device may not be an issue. But a malfunctioning device can put out a lot of RF interfeernece that a properly functioning device wouldn't, but the only sympton may be the greater RF intereference, and I bet you aren't checking for that symptom on your devices regularly, are you???