Les, the playing along with others has 2 problems:

1: You have to find that other.

2: Having found that other, you need to be sure they are playing in time.

Also, and just a thought. When people are playing live with people I think there is a certain amount of ability and or tendency of others to react to changes of tempo so everything holds together even if timing has drifted and perhaps sometimes none of the players would realise they had gone out against a metronome. So I'd guess if you really wanted to be strict, the mechanical device is the sure way.

Jon