The phone doesn't have to be plugged, the node doesn't have to be running 24/7. You only turn on the node when you will make a transaction, itcan sync to the latest block quickly enough.

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If it's not synced it can't relay and if it's not going to relay it's just a drain on the network.

So at scale, is it desirable?

It could remain active(relaying) while the phone is plugged/charging, a lot of apps do this.