I think a lot depends on how much discussion has gone on, how good the player is at reading people, and probably several other factors. I suspect that your assertion, Nigel, is true a lot less often than you think it is. Hell, half the time I'm not even sure what
I'm thinking, much less what my partner is!
I think when you're not sure what partner's call means, but think that it may be alertable, you should alert. In fact, the ACBL alert regulation says specifically "when in doubt, alert", which is probably closer to 1% confidence than 50% — although I wouldn't want to try to put a number on it (I think people rely too much on numbers for things like this, as if putting a number on it ropes and ties it — it doesn't).
Then there's the problem with the actual explanation. "I'm taking it as..." doesn't cut it. At the other end "undiscussed" doesn't cut it either when there is relevant experience or other information. The problem, as has been demonstrated somewhere here recently (maybe even in this thread) is knowing what's relevant, or perhaps more importantly what your opponents, the director and maybe the AC will consider relevant.
The worst thing we could do for the game though would be to treat "forgets" as harshly as Bobby Wolfe seems to want to do, at least at most levels. Maybe in world championships "convention disruption" should be dealt with harshly, but if you do that at club level, or even local tournament level, you'll just drive people away from the game — or else end up with a game where everyone plays the same vanilla system, there is no innovation, no differences. I don't know about anyone else, but that would bore the hell out of me. Might as well just play Whist.