There could of course be downsides to that.

1) in situations where phones are not meant to be switched on (e.g. some flights)

2) from a privacy perspective not being able to ensure your phone is actually off is not great.

You can't be sure that your phone is off already. Phones can be remotely activated and used as eavesdropping devices.

Is this really true or your speculation? Really curious about that but yeah, I am also suspecting that while the phones seem quiet they are actually listening ... sometimes.

In 2014 Snowden said NSA can do so (but not how).

It was quite discussed at that time. for example: http://www.tomsguide.com/us/nsa-remotely-turn-on-phones,news...

That's not even close - you linked to an article from 2006 (before the first iPhone was introduced) about microphone activation on phones like Motorola RAZRs which are already powered on. Oh, and this was legal and in the open.

Nice try, but that's a long way from a (long running, secret, and still hidden) vast government/corporate conspiracy to remotely power on any smartphone to spy on the owner.

Anyway, I guess a thief could just wrap the phone in tinfoil :)